Everyone has their own list of books amongst the main things we share between each other, such as the Andrew Loomis series, that have guided us, and given us invaluable information. So why not share these too, considering it's an often asked question?
Some of my own books are:
Drawing the Human Body by Sterling
Fantasy Art Workshop by John Howe (Very informative.)
Drawing Wrinkles and Drapery by Hogarth
Anatomy for the Artist by Tom Flint
and lots of random magazines for general referencing and learning.
In here, just for information sharing amongst us, and for those who are looking to expand their library, let's all list some books and sites that have helped us sharpen our painting, our sketches, compositional values, etc.