There are many books about writing and more than few focus on science fiction and fantasy. Here are a few I recommend:

Sometimes the Magic works: Lessons From a Writing Life by Terry Brooks
More inspirational than instructional, but still great. Brooks shares how he started his career and many things he has learned along the way.

Zen in the Art of Writing by Ray Bradbury
This book contains collected essays on writing. Bradbury tells how he finds inspiration and works to a story.

The Writers Complete Fantasy Reference
Although not a conclusive reference, this is a good starting point for a fantasy writer’s research.

How to Write Science Fiction and Fantasy by Orson Scott Card
Card offers advice for writers.

Writing the Breakout Novel and Writing the Breakout Novel Workbook by Donald Maass
Although not specifically for SF & fantasy writers, these companion books strive to help writer improve their novel writing abilities by identifying the qualities of successful books and showing you how to imcorporate them into your own work.

I will be adding resources as I find them so if you know of any I have missed, either email me or stop by the SF/Fantasy Books Forum