The Male Couple - How Relationships Develop, by Drew Mattison and David McWhirter. Buy it used here, here, here, here, here, here and here. Don't spend more than ten bucks - There are a lot of very good deals.
I give credit to this book for my nearly 23-year relationship. I consider it to be THE SHOP MANUAL for gay-male, long-term relationships. It's methodical, it's dry, but it's worth going through and STUDYING with every bit of your concentration, because it is the first scientific, clinical study of male-male relationships of durations between one year and over fifty years.
If you're in a relationship, it will help you relax more and accept the natural phases that all male-male relationships go through, and that are clearly explained in the book. If you're looking for a relationship that lasts, then this will help you start with a much more realistic viewpoint, with a high likelihood of succeeding.
This one has the "traditional" believers in monogamy FREAKED OUT, and when you read it, you can see why. It landed upon the sociology community like an atomic bomb, and there is even a small industry in new books written specifically to pick THIS book apart, and diminish its impact. It's really rattling the Judeo-Christian community's cage, big time. It completely discounts the religious strictures on sex, scientifically.
The authors make an absolutely solid case for monogamy being abnormal for humans. They assert that it was imposed upon us as a cultural artifact. They explain why your penis has a head on it: it is shaped to suction other men's semen out of the way so that yours has a better chance at fertilization during a gangbang. They go on and on, and NAIL the explanations.
It's a fun, easy read. Piss off a fundamentalist, and pass it along!
The Ethical Slut: A Practical Guide to Polyamory, Open Relationships & Other Adventures, by Dossie Easton and Janet W. Hardy
I haven't read this yet, but Magnum really likes it. I have the ebook and the audiobook. Time to start enjoying it!
Here is what it says on Amazon.com (you can buy it used, here):
For anyone who has ever dreamed of love, sex, and companionship beyond the limits of traditional monogamy, this groundbreaking guide navigates the infinite possibilities that open relationships can offer. Experienced ethical sluts Dossie Easton and Janet W. Hardy dispel myths and cover all the skills necessary to maintain a successful and responsible polyamorous lifestyle--from self-reflection and honest communication to practicing safe sex and raising a family. Individuals and their partners will learn how to discuss and honor boundaries, resolve conflicts, and to define relationships on their own terms.
"I couldn't stop reading it, and I for one identify as an ethical slut. This is a book for anyone interested in creating more pleasure in their lives . . . a complete guide to improving any style of relating, from going steady to having an extended family of sexual friends." --Betty Dodson, PhD, author of Sex for One