I have bought the following books to aid me in fantasy drawing and have found them indispensable
Keys to Drawing by Bert Dodson
Dracopedia by William O’ Conner (A study of mythical dragons and how to draw them)
Fantasy Art for Beginners (A great step by Step guide to drawing fantasy creatures)
How to paint and draw fantasy creatures by Imaginefx (Part of how to paint and draw series by Imaginefx)
I would specially recommend ImagineFX because it is the only magazine by famous CG and traditional artists for scifi genre. It contains a lot of good advice and workshops.