iTunes constantly crashes after adding books

Hey,

I have run into a big problem. iTunes constantly crashes now when I try to sync new books I have added to the iTunes library. I added roughly 4 GB of books in PDF file format. Right now I can't sync any book anymore. I already completely restarted my MacBook Air as well as reseted the iPhone I'm trying to sync with. Does anybody have an idea what might be causing this?

Thanks in advance!