You have 2 problems. The global $post value is not set at ‘init’ and this means that ACF does not know what post to get the value from.
The soonest you can to this is the ‘wp’ hook.
add_action( 'wp', 'redirect_product' );
The next thing you need to do is tell ACF what post to get the value from because it still can’t know because this is outside of “The Loop”
$queried_object = get_queried_object();
if (!is_a($queried_object, 'WP_Post')) {
// not a post
return;
}
$post_id = $queried_object->ID;
// I would also probably check the post type here
if (get_post_type($post_id) != 'My Post Type') {
return;
}
$redirect = get_field('linked_book', $post_id);
