How do I enforce a specific custom post type on the loop

You can modify all main queries before they happen via the pre_get_posts action and a check if is_main_query:

function wpa75492_post_type_query( $query ) {
    if ( $query->is_main_query() ) {
        $query->set( 'post_type', array( 'book' ) );
    }
}
add_action( 'pre_get_posts', 'wpa75492_post_type_query' );

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