Books were hard to part with, but I did it, too. Actually, the easiest books to donate were CS and math, especially programming related books. I refuse to part with my copies of CLR and Common LISP, though. In the end, I shed a couple hundred books and it was cathartic, kind of like start over with a beginner's mind.

P.S. Local universities are great places to make instant friends when you have books to donate.