My absolute favorite poly book has to be "What Does Polyamory Look Like?"
by Mim Chapman (first referenced in Alan7388's top-10 list in this thread).
This book was instrumental in helping me recognize and broaden my understanding of various different types of poly relationships, from pair-based relationships to inwardly-focused groups, chains (V,N,W,...,S), etc. It's not an "apologetics"-style of book, i.e. it's not trying to justify or rationalize polyamory. Rather, it's focused on giving a practical advise to people that are already poly but trying to understand what types of relationships suit them best.
For me, it was the right book at the right time. It also helped me and my wife to draft our very first "poly agreement", and to do a MUCH BETTER job at it than we would have otherwise! While many of the concepts may be obvious to those that have been poly for many years, I highly recommend it to anyone that considers themselves relatively new to polyamory (like me).