The Ethical Slut
I'm quite curious about the breadth of possibilities in living life and this book was a refreshingly clear and proud discussion of a traditionally taboo part of life -- having multiple partners. Despite the fact that most people at some point in their life have had more than one love or lust interest on the go at the same time, there has been surprisingly little written about how to embrace this in a healthy way. I've heard about "open relationships", but have always wondered how they actually work (or not work). This book gets into the details. It's hard to say how accurate or helpful this book really is -- I haven't practiced the writings myself, and know of very few successful attempts at polyamoury. Have you? It sounds like you need to have a pretty solid primary relationship and be quite emotionally stable, communicative and honest to make a good go of polyamoury.
But if you've ever been in love with more than one person at a time, I highly recommend this book. There's a chapter on understanding jealousy that is worth the price of the book alone.