Art books and reference books (mainly things to do with CG, animation, and lighting). My most favourite art book is the Riven book. It's absolutely beautiful. It's a shame Cyan Worlds and Ubisoft have stopped the Myst franchise, it had such gorgeous and majestic designs and art directions.

Anyone who is into art books and making of (especially with a lot of nurbs modelling/texturing insights), get this book. It's expensive brand new, but a lot of good quality second hand versions are affordable.