Don't delete them, but add a clause in the original user agreement that after six months of inactivity, a user will be " retired" When that happens, the book becomes private, and any books that they are backing are removed from their shelf.
By making them private instead of deleting them, they will be able to come back later and re-activate their account if they should wish, without going through the hassle of re-uploading etc.

Dadoo is the voice of reason. Always. I want to be like Dadoo if I ever grow up.