My list of Poly books that I think are worth reading, pretty much in order that I think they are worth reading in:

More Than Two You will find that this book is pretty much universally regarded as the best intro book into Polyamory.

The Usual Error After reading More Than Two you have a bit of a choice. Good communication skills is the most important thing you need to learn to be successful at poly and this book will teach you the basics. If you feel that you are already a good communicator I would probably say skip this book for now. But if you feel that there are common misunderstandings and miscommunications between you and your partner(s), or still think that men come from Mars or women from Venus, then start here. It is pretty basic communication skills book. I would come back to it eventually, because it does have some really good stuff in it even for a good communicator.

Nonviolent Communication: A Language of Life After you learn what More Than Two can teach you about Polyamory you should read this book. It is hands down the best book on how to effectively have those hard emotional conversations without fighting.

That is really it. Those three books will give you a good start. After that I would recommend the following books if you are still looking to do more reading on the subject:

The Ethical Slut This book is more philosophical then the others. It is really about free love style of nonmonogamy rather than strictly polyamory. It has some good lessons in it, and is rather helpful if you are still trying to shed the 'sex is shameful' mindset, but I don't think it taught me much about how to actually do polyamory.

Opening Up It is a good book. It is just not quite as good as More Than Two, and really the two of them have pretty much the same information. Read it when you get around to it, but once you have read one of the two poly books (More Than Two and Opening Up) then your time would probably be better spent reading the books on communication.

Avoid Sex at Dawn. It is a interesting book but it tries to pass itself off as science when in truth it's claims are very weak and not well supported by research. It makes a lot of assumptions that have no real basis in science and are really mostly his own opinion. It is a popular book among non-monogamous people mostly because we agree with those assumptions, and it tells us what we want to hear, but try to remember that agreeing with assumptions is not proof of them.