I have 3 bookcases too (15 shelves and a bit, to be exact). All my books are neatly on shelves, I would never keep them in boxes for long unless I ran totally out of space ... which I have mostly been for a while, but I keep making room somehow. Many of my books are quite big, and expensive (being textbooks) however, so if you have 3 bookcases of fairly small books, that's easily more than 400, I'd guess, plus whatever you have in boxes.