I just posted this in response to Todfox's request, and re-posting them here seemed topical.
...to indulge my unhealthy penchant for delaying my entry into bed even LONGER.
I'm sure many of these are on major lists, but I've read them all personally at least once and found them helpful:
Redefining Our Relationships: Guidelines for Responsible Open Relationships
A very good poly primer. Short, useful, covers all the bases, including the controversial idea that parents don't necessarily have to be devoted to each other as long as they are devoted to their child.
Nonviolent Communication - A Language of Life by Marshal B. Rosenberg, Ph. D.
Ever wonder how to communicate strong feelings with making it seem like an attack, or to feel heard and help others do the same? This book helps identify negative patterns of communication and shows positive alternatives. Recommended by Poly Paradise at Burning Man.
Choice Theory and other books by William Glasser
The most effective self-book series I've ever read. Learn to identify the dysfunctional control theory we are all taught by society, our parents, etc., and learn how to break the cycle and stop trying to control other people. A must for "I know what's good for you" people and those with big jealousy issues. I was amazed at how fast I was able to see these patterns in myself, how fast I could choose to work on changing them, and how immediate the results were.
Gay Relationships by Tina Tessina
Mainly poly-friendly because so many gay relationships are, the gay bent of this book tends to take out a lot of the monogamocentric bias almost always present in relationship books, and provides good, clear-headed, step-by-step advise for anyone involved in intimate relationships.
Lesbian Ethics -- Toward New Value by Sara Lucia Hoagland
Extremely valuable re-interpretation of many fundamental aspects of relationships from an outside perspective. The book could not have been developed without a separatist community, but the perspectives and advise are almost all universal.
Survivors of an Open Marriage - Jennifer Gates
A rare counter-example of polyamorous relating. Story of a married couple who tried polyamory and chose eventually to be monogamous. Worth reading both for the reality check and also as a poly "what did they do wrong and how could they have made it work?" skillset challenge. Great for group discussion.
Men Are From Mars, Women Are From Venus - John Gray, Ph.D.
I can hear the groans already. :) Although this book has both a slight monogamous and sexist bias, it's still invaluable both as a way to understand the things that, for whatever reason, seem to be fundamentally different between the sexes, and also for valuable advise on relating and communicating.
The New Intimacy -- Open Ended Marriage and Alternative Lifestyles by Ronald Mazur
Very pithy for it's small size. A very good book for people who are already in a long-term monogamous relationship, but are looking at branching out and including other people. Covers common pitfalls and solutions, including things like identifying different kinds of jealousy, etc. A good first book for hesitant but interested couples. Look at poly as an evolution of consciousness, which I identify with strongly.
Polyamory The New Love without Limits -- Secrets of Sustainable Initimate Relationships by Deborah M. Anapol
I like the focus of this book on the idea that poly relationships are the solution to the problem of unsustainability that often occurs in monogamous relationships. One reason I've always identified as poly is because I recognized early the inherent pitfalls of monogamy and knew they would likely take away the people I loved unless I found an alternative. Addresses poly identity, jealousy, networking, poly myth, and resonant personal anecdotes.
And, of course:
The Ethical Slut - A Guide to Infinite Sexual Possibilities
The first major poly book I read where I finally realized I'd found my tribe. Lots of good how-to advise from people who have ever so been there. Gets lots of street cred from women who think that poly is some male plot to get laid a lot (it is, but it's a female plot too. :) ) A little more refreshingly in-your-face than most poly books. I've given out at least 8 copies of this now, and need to find a way to write it off my taxes.
I own at least one copy of all of these books and am willing to loan them out. If I don't get them back, our relationship may not be sustainable. :)