I can kindof understand coloring books and postcards...I guess...but why are calendars not allowed?  Or a workbook?  If they're following PBS guidelines and there is no writing in the "book", is there a reason why they aren't allowed...or just that they aren't allowed because they aren't "books"? :). 

Workbooks are allowed, as long as they have an ISBN and meet the other posting guidelines. If a workbook has been written in, there is a special rule about notifying the receiver and getting their acceptance of the writing in the workbook BEFORE it is mailed. If a workbook has no writing, then no PM is needed.

Calendars are not books.  Simple enough.