Websites with information about sex therapy:
The College of Sexual and Relationship Therapists (COSRT) provides detailed information about sex therapy.
Relate also provides a good introduction.
Websites with information about sexual problems:
The Sexual Advice Association has a series of advice sheets on most sexual problems. The section on erectile difficulties is particularly good.
The Channel 4 'sexperience' site is not just for teenagers! It has a lot of useful information about sexual functioning and common sexual concerns.
The Relate Guide to Sex in Loving Relationships by Sarah Livitnoff is a good introduction to sex therapy, and is packed full of case studies and information for couples.
The New Male Sexuality by Bernie Zilbergeld explodes common myths about male sexuality, and gives practical advice about how to address common sexual problems experienced by men.