Registering a Custom Template with the WP AMP plugin

wp-amp-tutorial-register-templates.php

<?php
/**
 * Add a custom AMP template file.
 *
 * Registers templates in a templates subdirectory by post type slug.
 * (e.g., "./templates/book.php")
 *
 * @filter amp_post_template_file
 *
 * @param string $file The file name input.
 * @param string $type The type of template.
 * @param object $post WP Post Object.
 *
 * @return string
 */
public function xwp_amp_custom_templates( $file, $type, $post ) {
	if ( 'single' === $type && 'book' === $post->post_type ) {
		$file = dirname( __FILE__ ) . '/templates/book.php';
	} elseif ( 'single' === $type ) {
		$file = dirname( __FILE__ ) . '/templates/single.php';
	}
	return $file;
}
add_filter( 'amp_post_template_file', 'xwp_amp_custom_templates', 10, 3 );