The Ethical Slut — Currently the reigning bible of poly relationships, a required read, but someone lacking in some areas in my opinion.
The Art of Happiness — Taking poly to it’s extreme, love everyone. Seriously, if you want to be truly happy, even if you have zero interest in buddhism, read this book.
Why Marriages Succeed or Fail — Picking up here the Ethical Slut leaves off, this book covers communication and what it takes to make a relationship succeed long term. Based on a long term scientific study of marriages it goes beyond assumption and explains the dynamics of being happily married.
Men Are From Mars, Women are From Venus – Another exceptional book on communication and understanding the differences between men and women and how they communicate and give and receive love.