Abbys-Sexual-Health.com: Masturbation
Has excellent articles on both male and female masturbation, among other sexual health topics.
http://www.abbys-sexual-health.com/

Clitical
A good web site about female sexuality, including anatomy (with pictures), female masturbation, sex toy reviews, and other sex topics
http://www.clitical.com/

Jackinworld.com
Very comprehensive site on male masturbation
http://jackinworld.com

The Koi Sex Tutorials: Masturbation
Good basic information on female and male masturbation techniques as well as masturbation for two.
http://very-koi.net/projects.htm

Let’s Masturbate
Masturbation tips and techniques
http://www.letsmasturbate.com/

Sex For One: The Joy of Selfloving by Betty Dodson, published by Three Rivers Press [Buy]
For more than 20 years, Betty Dodson has been dedicated to taking the shame out of masturbation, showing it to be a healthy form of sexual expression. With warmth and intelligence, and informative line drawings, Dodson explains how anyone can learn to fully enjoy the pleasures of self-love, pointing out that masturbation is still the safest sex.
ISBN 0517886073

Sex Tutor: Masturbation
Tips on technique for female and male masturbation.
http://www.sextutor.com/

The-Clitoris.com
A site dedicated to women’s sexual pleasure and health, with an emphasis on pleasure anatomy.
http://the-clitoris.com/