Here are the questions to ask...
1) Which books have I not touched for 5 years?
2) Which books have I not touched since I bought them?
3) Which books do I no longer need?
4) Which books are on topics that are easy to find on the web or research in a bookstore?
Next, look at your shelves...
5) How full are they, are they more than 2/3 full?
(2/3 allows room for new books
6) Do you need room for other books?
7) Could you use the space where the bookshelves are for another purpose?
8) Are the books collecting dust such that they might cause you to get sick?
9) Are there books that stress you when you pass by? (don't keep them)