I wrote my own system that started out as a book catalogue but morphed
to encompass archive items and physical objects.
I allow hierarchy by allowing some thing to be enclosed in another
even though it is in one table and use the code to find this if
needed.
It also has the concept of subjects, of a book, of a picture etc
it is easy to tie stuff together by abusing the dewey field eg 621.833
will get all books, manufacturers catalogues, national standards etc
on gear cutting
Best not get to religious on categories from any one area

Dave Caroline