Sex At Dawn: How We Mate, Why We Stray, and What It Means for Modern Relationships
by Christopher Ryan and Cacilda Jetha

This is not, strictly speaking, a book about polyamory. Instead, it's a wildly entertaining, meticulously researched tretise on the origins of human sexuality and the things that make us relate the way we do. The authors challenge the assumptions of monogamy as a norm for human relationships, in a provocative and fun-to-read way.

Open All the Way: Confessions From My Open Marriage
by Sadie Smythe (Kindle only)

Sadie Smythe is a well-known blogger who writes openly, frankly, and compellingly about her experiences in opening her long-term marriage. In this book, she documents the ups and downs, the lessons, th pitfalls, and the happiness of her own journey from monogamy to non-monogamy. This entertaining book is good reading for anyone seeking to open their own marriage.

The Art and Etiquette of Polyamory: A Hands-on Guide to Open Sexual Relationships
by Françoise Simpère

Another perosnal narrative, this book describes the author's process to coming to her own polyamorous arrangement, and talks about the rules and ideas that keep her relationships healthy and happy. It's written from a very specific perspective (long-term couples who want lovers on the side), and as such describes only one particular kind of polyamory. It offers practical suggestins and how-tos for couples looking to arrange non-monogamous relationships.

Opening Up: A Guide to Creating and Sustaining Open Relationships
by Tristan Taormino

Part anecdote, part interviews with poly couples, part advice, this wonderful book is an outstanding answer to the question "So how do you make non-monogamy work, anyway?" Through interviews with folks practicing a wide variety of different forms of non-monogamy, this book covers several styles of polyamory, swinging, and even polyamory for single people. There's a companion blog as well.

The Ethical Slut: A Guide to Infinite Sexual Possibilities by Dossie Easton and Catherine Lizst

Considered by many people to be one of the definitive guides to "responsible non-monogamy." While it does not really discuss polyamory per se, but rather polysexuality--the practice of multiple sexual partners in an ethical framework--this book is a good read for anyone interested in polyamory. Published by Greenery Press.

Open: Love, Sex, and Life in an Open Marriage
by Jenny Block

This book is a personal narrative of one "average girl next door" who just so happens to be in an open relationship. The book covers the ins and outs of her own relationship, the lessons she's learned along the way, and examines why its still more socially acceptable to cheat than to be polyamorous.

Polyamory: Roadmaps for the Clueless & Hopeful
by Anthony Ravenscroft

Written with humor, wit, and style by a person with twenty years' experience in polyamorous relationships, this book is a guidebook to the benefits, pitfalls, mistakes, and rewards of a poly life. Covers many of the mistakes the author has made without flinching, and lays out a roadmap for successful polyamorous relationships.

The Sex And Love Handbook: Polyamory! Bisexuality! Swingers! Spirituality! & Even Monogamy! A Practical Optimistic Relationship Guide by Kris A. Heinlein and Rozz M. Heinlein

This book, an overview of many relationship styles and sexual practices, explores the emotions, philosophies, risks and rewards of non-traditional sexual and romantic relationships. Written in an optimistic, enthusiastic style, it presents a wide range of relationship models engagingly and honestly.

Redefining Our Relationships: Guidelines for Responsible Open Relationships by Wendy-O Matik

This book explores the realities of day-in, day-out non-monogamy, particularly as a conscious political and social act. It's direct, in your face, and offers excellent insight on managing problems like jealousy.

The Myth of Monogamy: Fidelity and Infedelity in Animals and People by David P. Barash and Judith Eve Lipton

Written by a husband and wife team of behavioral scientists, this book explores non-monogamy from a behavioral, genetic, and moral standpoint. The book is not hostile to monogamy, but it makes the statement that in humans, relationship model is a matter of both biology and choice, and that monogamy is not more inherently "natural" than non-monogamy.

Polyamory in the 21st Century: Love and Intimacy with Multiple Partners
by Deborah Anapol

A followup to Polyamory: The New Love Without Limits (listed below), this book is a retrospective on Dr. Anapol's life as a poly activist. While it has some sections that are likely to be useful to newcomers, this isn't really a book aimed at those new to poly; instead, it's a collection of Dr. Anapol's personal experiences in polyamorous relationships.

The New Intimacy: Open-Ended Marriage and Alternative Lifestyles
by Rev.Ronald M. Mazur

This book advertises itself as "for non-traditionalists, those ready for new life and love affirmations." The Rev. Dr. Ron Mazur has been an ordained minister, a university health educator, a clinical sexologist in private practice, and a pioneer in computer on-line networks.

The Lesbian Polyamory Reader
by Marcia Munson and Judith P. Stelboum (Editors)

A collection of stories, poetry, and essays on non-monogamous lesbian relationships, together with descriptions of solid frameworks for ethical non-monogamy.

Monogamy
by Adam Phillips

Written by a psychologist, this book examines the traditional concept of marriage in a society where divorce is altogether common.

Polyamory: The New Love Without Limits by Dr. Deborah T. Anapol

This book focuses more on polyfidelity than on other kinds of polyamory, with a heavy emphasis on New Age spirituality, and is often recommended by those who combine polyamory and spirituality.

The Seven Natural Laws of Love by Dr. Deborah T. Anapol

This book draws from thousands of interviews to try to create a set of rules which govern all loving relationships regardless of their form.

Lesbian Polyfidelity: A Pleasure Guide for all Women Whose Hearts are Open to Multiple Sexual Loves by Celeste West

Recommended by an online visitor; I don't know much about this book personally, but it's received excellent reviews.