I was looking at the database tonight and wrote some queries that look for problem entries in the database. I suspect that any rows returned can be deleted but I have not tested it so you are on your own.
Looks for entries in the Content table that are not associated with a book entry
Code:
select * from Content where BookID not in (select contentid from content where BookID is Null )
Looks for entries in volume_shortcovers that have no related books
Code:
select * from volume_shortcovers where volumeid not in (select contentid from content where BookID is Null )
Tim