I'm using the book module where I have books and pages. I created a content type 'category' where you only need a title. The intention is that you can only add pages (with content) underneath those 'categories'.
I tried using the hook_form_FORM_ID_alter() where I altered the first select box, because the second select box works with an ajax callback I can not alter it.
Is there a way to restrict the second select box to only see types of 'category'?