1. "How to render" by Scott Robertson.
Scott Robertson is my favorite author. First, he is one of the best instructors in the world. His books are very well thought out.
2. Anatomy for sculptors.
I have a couple of books on anatomy and this new one is amazing. Its price is the only drawback.
3. 3DTotals' "Sketching from the imagination".
Other people's sketches always inspire.