add_action( 'rest_api_init', 'register_custom_rest_routes' );
function register_custom_rest_routes() {
    // Namespace: myplugin/v1 (enables versioning)
    $namespace = 'myplugin/v1';

    // GET /wp-json/myplugin/v1/books
    register_rest_route( $namespace, '/books', [
        'methods'  => 'GET',
        'callback' => 'get_books_callback',
        'permission_callback' => '__return_true', // Public endpoint
        'args' => [
            'per_page' => [
                'default' => 10,
                'validate_callback' => function( $param ) {
                    return is_numeric( $param ) && $param > 0 && $param <= 100;
                },
                'sanitize_callback' => 'absint',
            ],
            'page' => [
                'default' => 1,
                'validate_callback' => function( $param ) {
                    return is_numeric( $param ) && $param > 0;
                },
                'sanitize_callback' => 'absint',
            ],
        ],
    ]);

    // GET /wp-json/myplugin/v1/books/(?P<id>\d+)
    register_rest_route( $namespace, '/books/(?P<id>\d+)', [
        'methods'  => 'GET',
        'callback' => 'get_book_callback',
        'permission_callback' => '__return_true',
        'args' => [
            'id' => [
                'validate_callback' => function( $param ) {
                    return is_numeric( $param );
                },
                'sanitize_callback' => 'absint',
            ],
        ],
    ]);

    // POST /wp-json/myplugin/v1/books (authenticated)
    register_rest_route( $namespace, '/books', [
        'methods'  => 'POST',
        'callback' => 'create_book_callback',
        'permission_callback' => function() {
            return current_user_can( 'edit_posts' );
        },
        'args' => [
            'title' => [
                'required' => true,
                'type' => 'string',
                'validate_callback' => function( $param ) {
                    return is_string( $param ) && strlen( $param ) > 0;
                },
                'sanitize_callback' => 'sanitize_text_field',
            ],
            'content' => [
                'required' => false,
                'type' => 'string',
                'sanitize_callback' => 'wp_kses_post',
            ],
            'status' => [
                'default' => 'draft',
                'enum' => [ 'draft', 'publish', 'private' ],
            ],
        ],
    ]);

    // PUT /wp-json/myplugin/v1/books/(?P<id>\d+)
    register_rest_route( $namespace, '/books/(?P<id>\d+)', [
        'methods'  => 'PUT',
        'callback' => 'update_book_callback',
        'permission_callback' => function( $request ) {
            $book_id = $request->get_param( 'id' );
            return current_user_can( 'edit_post', $book_id );
        },
        'args' => [
            'id' => [
                'validate_callback' => function( $param ) {
                    return is_numeric( $param );
                },
                'sanitize_callback' => 'absint',
            ],
            'title' => [
                'type' => 'string',
                'sanitize_callback' => 'sanitize_text_field',
            ],
            'content' => [
                'type' => 'string',
                'sanitize_callback' => 'wp_kses_post',
            ],
        ],
    ]);

    // DELETE /wp-json/myplugin/v1/books/(?P<id>\d+)
    register_rest_route( $namespace, '/books/(?P<id>\d+)', [
        'methods'  => 'DELETE',
        'callback' => 'delete_book_callback',
        'permission_callback' => function( $request ) {
            $book_id = $request->get_param( 'id' );
            return current_user_can( 'delete_post', $book_id );
        },
        'args' => [
            'id' => [
                'validate_callback' => function( $param ) {
                    return is_numeric( $param );
                },
                'sanitize_callback' => 'absint',
            ],
        ],
    ]);
}

// GET /wp-json/myplugin/v1/books
function get_books_callback( $request ) {
    $per_page = $request->get_param( 'per_page' );
    $page = $request->get_param( 'page' );
    $offset = ( $page - 1 ) * $per_page;

    $args = [
        'post_type' => 'book',
        'posts_per_page' => $per_page,
        'offset' => $offset,
        'post_status' => 'publish',
    ];

    $query = new WP_Query( $args );

    if ( ! $query->have_posts() ) {
        return rest_ensure_response([
            'books' => [],
            'total' => 0,
            'page' => $page,
            'per_page' => $per_page,
        ]);
    }

    $books = [];
    while ( $query->have_posts() ) {
        $query->the_post();
        $books[] = [
            'id' => get_the_ID(),
            'title' => get_the_title(),
            'content' => get_the_content(),
            'author' => get_the_author(),
            'date' => get_the_date( 'c' ), // ISO 8601 format
            'link' => get_permalink(),
        ];
    }
    wp_reset_postdata();

    $response = rest_ensure_response([
        'books' => $books,
        'total' => $query->found_posts,
        'page' => $page,
        'per_page' => $per_page,
        'total_pages' => ceil( $query->found_posts / $per_page ),
    ]);

    // Add HATEOAS links
    $response->add_link( 'self', rest_url( "myplugin/v1/books?page={$page}&per_page={$per_page}" ) );

    if ( $page > 1 ) {
        $prev_page = $page - 1;
        $response->add_link( 'prev', rest_url( "myplugin/v1/books?page={$prev_page}&per_page={$per_page}" ) );
    }

    if ( $page < ceil( $query->found_posts / $per_page ) ) {
        $next_page = $page + 1;
        $response->add_link( 'next', rest_url( "myplugin/v1/books?page={$next_page}&per_page={$per_page}" ) );
    }

    return $response;
}

// GET /wp-json/myplugin/v1/books/123
function get_book_callback( $request ) {
    $book_id = $request->get_param( 'id' );
    $book = get_post( $book_id );

    if ( ! $book || 'book' !== $book->post_type ) {
        return new WP_Error(
            'book_not_found',
            'Book not found',
            [ 'status' => 404 ]
        );
    }

    $data = [
        'id' => $book->ID,
        'title' => $book->post_title,
        'content' => apply_filters( 'the_content', $book->post_content ),
        'excerpt' => $book->post_excerpt,
        'author' => get_the_author_meta( 'display_name', $book->post_author ),
        'date' => get_the_date( 'c', $book ),
        'modified' => get_the_modified_date( 'c', $book ),
        'status' => $book->post_status,
        'link' => get_permalink( $book ),
        'featured_image' => get_the_post_thumbnail_url( $book, 'large' ),
        'meta' => [
            'isbn' => get_post_meta( $book->ID, '_isbn', true ),
            'pages' => (int) get_post_meta( $book->ID, '_pages', true ),
        ],
    ];

    return rest_ensure_response( $data );
}

// POST /wp-json/myplugin/v1/books
function create_book_callback( $request ) {
    $title = $request->get_param( 'title' );
    $content = $request->get_param( 'content' );
    $status = $request->get_param( 'status' );

    $post_data = [
        'post_type' => 'book',
        'post_title' => $title,
        'post_content' => $content,
        'post_status' => $status,
        'post_author' => get_current_user_id(),
    ];

    $book_id = wp_insert_post( $post_data, true );

    if ( is_wp_error( $book_id ) ) {
        return new WP_Error(
            'book_creation_failed',
            $book_id->get_error_message(),
            [ 'status' => 500 ]
        );
    }

    // Add custom metadata if provided
    if ( $request->has_param( 'isbn' ) ) {
        update_post_meta( $book_id, '_isbn', sanitize_text_field( $request->get_param( 'isbn' ) ) );
    }

    if ( $request->has_param( 'pages' ) ) {
        update_post_meta( $book_id, '_pages', absint( $request->get_param( 'pages' ) ) );
    }

    $response = rest_ensure_response([
        'id' => $book_id,
        'title' => $title,
        'message' => 'Book created successfully',
        'link' => get_permalink( $book_id ),
    ]);

    $response->set_status( 201 ); // Created
    $response->header( 'Location', rest_url( "myplugin/v1/books/{$book_id}" ) );

    return $response;
}

// PUT /wp-json/myplugin/v1/books/123
function update_book_callback( $request ) {
    $book_id = $request->get_param( 'id' );
    $book = get_post( $book_id );

    if ( ! $book || 'book' !== $book->post_type ) {
        return new WP_Error(
            'book_not_found',
            'Book not found',
            [ 'status' => 404 ]
        );
    }

    $post_data = [ 'ID' => $book_id ];

    if ( $request->has_param( 'title' ) ) {
        $post_data['post_title'] = $request->get_param( 'title' );
    }

    if ( $request->has_param( 'content' ) ) {
        $post_data['post_content'] = $request->get_param( 'content' );
    }

    if ( $request->has_param( 'status' ) ) {
        $post_data['post_status'] = $request->get_param( 'status' );
    }

    $result = wp_update_post( $post_data, true );

    if ( is_wp_error( $result ) ) {
        return new WP_Error(
            'book_update_failed',
            $result->get_error_message(),
            [ 'status' => 500 ]
        );
    }

    return rest_ensure_response([
        'id' => $book_id,
        'message' => 'Book updated successfully',
        'link' => get_permalink( $book_id ),
    ]);
}

// DELETE /wp-json/myplugin/v1/books/123
function delete_book_callback( $request ) {
    $book_id = $request->get_param( 'id' );
    $book = get_post( $book_id );

    if ( ! $book || 'book' !== $book->post_type ) {
        return new WP_Error(
            'book_not_found',
            'Book not found',
            [ 'status' => 404 ]
        );
    }

    // Soft delete (trash) or hard delete
    $force = $request->get_param( 'force' );
    $result = wp_delete_post( $book_id, $force );

    if ( ! $result ) {
        return new WP_Error(
            'book_deletion_failed',
            'Failed to delete book',
            [ 'status' => 500 ]
        );
    }

    return rest_ensure_response([
        'deleted' => true,
        'id' => $book_id,
        'message' => $force ? 'Book permanently deleted' : 'Book moved to trash',
    ]);
}

<?php
namespace MyPlugin\API;

class Books_Controller extends \WP_REST_Controller {

    protected $namespace = 'myplugin/v1';
    protected $rest_base = 'books';

    public function register_routes() {
        register_rest_route( $this->namespace, '/' . $this->rest_base, [
            [
                'methods'  => \WP_REST_Server::READABLE,
                'callback' => [ $this, 'get_items' ],
                'permission_callback' => [ $this, 'get_items_permissions_check' ],
                'args' => $this->get_collection_params(),
            ],
            [
                'methods'  => \WP_REST_Server::CREATABLE,
                'callback' => [ $this, 'create_item' ],
                'permission_callback' => [ $this, 'create_item_permissions_check' ],
                'args' => $this->get_endpoint_args_for_item_schema( \WP_REST_Server::CREATABLE ),
            ],
            'schema' => [ $this, 'get_public_item_schema' ],
        ]);

        register_rest_route( $this->namespace, '/' . $this->rest_base . '/(?P<id>[\d]+)', [
            'args' => [
                'id' => [
                    'description' => 'Unique identifier for the book',
                    'type' => 'integer',
                ],
            ],
            [
                'methods'  => \WP_REST_Server::READABLE,
                'callback' => [ $this, 'get_item' ],
                'permission_callback' => [ $this, 'get_item_permissions_check' ],
                'args' => [
                    'context' => $this->get_context_param( [ 'default' => 'view' ] ),
                ],
            ],
            [
                'methods'  => \WP_REST_Server::EDITABLE,
                'callback' => [ $this, 'update_item' ],
                'permission_callback' => [ $this, 'update_item_permissions_check' ],
                'args' => $this->get_endpoint_args_for_item_schema( \WP_REST_Server::EDITABLE ),
            ],
            [
                'methods'  => \WP_REST_Server::DELETABLE,
                'callback' => [ $this, 'delete_item' ],
                'permission_callback' => [ $this, 'delete_item_permissions_check' ],
                'args' => [
                    'force' => [
                        'type' => 'boolean',
                        'default' => false,
                        'description' => 'Whether to bypass trash and force deletion',
                    ],
                ],
            ],
            'schema' => [ $this, 'get_public_item_schema' ],
        ]);
    }

    public function get_items( $request ) {
        // Implementation similar to get_books_callback above
    }

    public function get_item( $request ) {
        // Implementation similar to get_book_callback above
    }

    public function create_item( $request ) {
        // Implementation similar to create_book_callback above
    }

    public function update_item( $request ) {
        // Implementation similar to update_book_callback above
    }

    public function delete_item( $request ) {
        // Implementation similar to delete_book_callback above
    }

    public function get_items_permissions_check( $request ) {
        return true; // Public endpoint
    }

    public function get_item_permissions_check( $request ) {
        return true; // Public endpoint
    }

    public function create_item_permissions_check( $request ) {
        return current_user_can( 'edit_posts' );
    }

    public function update_item_permissions_check( $request ) {
        $book_id = $request->get_param( 'id' );
        return current_user_can( 'edit_post', $book_id );
    }

    public function delete_item_permissions_check( $request ) {
        $book_id = $request->get_param( 'id' );
        return current_user_can( 'delete_post', $book_id );
    }

    public function get_public_item_schema() {
        if ( $this->schema ) {
            return $this->add_additional_fields_schema( $this->schema );
        }

        $schema = [
            '$schema'    => 'http://json-schema.org/draft-04/schema#',
            'title'      => 'book',
            'type'       => 'object',
            'properties' => [
                'id' => [
                    'description' => 'Unique identifier for the book',
                    'type'        => 'integer',
                    'context'     => [ 'view', 'edit', 'embed' ],
                    'readonly'    => true,
                ],
                'title' => [
                    'description' => 'The book title',
                    'type'        => 'string',
                    'context'     => [ 'view', 'edit', 'embed' ],
                    'required'    => true,
                ],
                'content' => [
                    'description' => 'The book content',
                    'type'        => 'string',
                    'context'     => [ 'view', 'edit' ],
                ],
                'status' => [
                    'description' => 'The book status',
                    'type'        => 'string',
                    'enum'        => [ 'draft', 'publish', 'private' ],
                    'context'     => [ 'view', 'edit' ],
                ],
            ],
        ];

        $this->schema = $schema;

        return $this->add_additional_fields_schema( $this->schema );
    }
}

// Register controller
add_action( 'rest_api_init', function() {
    $controller = new \MyPlugin\API\Books_Controller();
    $controller->register_routes();
});