The Ethical Slut: A Guide to Infinite Possibilities by Dossie Easton and Catherine A. Liszt [Buy]
The authors provide a non-judgmental look at multiple-partner lifestyles and discuss practical advice for making it work with the right attitudes and communication skills and finding what works for everyone involved.
ISBN 1890159018

Loving More
National Support Organization/Magazine for Those Involved in Multi-Partner Loving Relationships
http://www.lovemore.com/

National Coalition for Sexual Freedom
National advocacy on sex issues, primarily focused on the rights of consenting adults in the SM-leather-fetish, swing, and polyamory communities, who often face discrimination because of their sexual expression.
http://www.ncsfreedom.org/

Polyamory Index
Lots of links and resources
http://www.polyamory.org/Howard

Polyamory.org
USENET’s alt.polyarmory resource center (see also alt.polyamory)
http://www.polyamory.org

Sacred Space Institute
Workshops and Conferences on Polyamory and Tantra
(415) 507-1739
http://www.lovewithoutlimits.com/