wp-amp-tutorial-register-templates-style-cpts.php

<?php
/**
 * Add custom AMP template files.
 *
 * Registers templates in a /templates subdirectory.
 *
 * @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
 */
function jr3_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';
	} elseif ( 'style' === $type ) {
	  	$file = dirname( __FILE__ ) . '/templates/style.php';
	}
	return $file;
}
add_filter( 'amp_post_template_file', 'jr3_amp_custom_templates', 10, 3 );