Passionate Marriage: Keeping Love and Intimacy Alive in Committed Relationships Amy's review: David Schnarch's Passionate Marriage looks at the importance of differentiation in relationships and marriage. Holding onto yourself in relationship to others is one of his basic tenants.
The Seven Principles for Making Marriage Work: A Practical Guide from the Country's Foremost Relationship Expert This book is very "readable". The authors include relationship examples and practical exercises to illustrate (and allow you to practice) the principles. I recommend this to couples (married or not) for them to get a sense of where their relationship is at (take the "relationship temperature", if you will) and where they might focus their time and energy. Often it is very small, incremental changes that make all the differences over the lifetime of any relationship. John Gottman also has other books worth reading.

The New Rules of Marriage: What You Need to Know to Make Love Work This book is on my read list for 2012.
Hold Me Tight: Seven Conversations for a Lifetime of Love This is another book I recommend to couples. In fact, I think this book is great for any relationship. It too is very "readable". I think with any book like this each reader has to take what works for them from the book and leave the rest behind. This book has practical exercises that encourage thought about relationships as well as action steps towards making changes in how you are in relationship to others.
Mindsight: The New Science of Personal Transformation Dan Siegel is one of my favorite speakers and authors. This is a great book for anyone who is interested in the science of relationships. Dr. Siegel coined the term "interpersonal neurobiology".
All the Rules: Time-tested Secrets for Capturing the Heart of Mr. Right All the Rules: Time-tested Secrets for Capturing
Coming Apart: Why Relationships End and How to Live Through the Ending of Yours While this book is about how to be with ending a relationship, it is also about what's next. How to care for yourself as you examine your own role in why your relationship ended. This book can help you identify what you might do (and/or how you might be) differently when you are ready to "get back out there".
Blessed by Thunder: Memoir of a Cuban Girlhood Blessed by Thunder was written by my dear friend and colleague, Flor Fernandez. I could not put this book down. It is enchanting. I hope you will give yourself the gift of reading her story. Set in Cuba and later in America, what comes across so powerfully are the connections Flor had and has with people in her life.
Interpersonal Edge: Breakthrough Tools for Talking to Anyone, Anywhere, About Anything Daneen Skube's book is wonderful particularly for those struggling with difficult relationships at work. And, her tools are applicable to any relationship. If you have ever struggled with boundaries in a relationship then I suggest this book for you. She has gentle, respectful and powerful tools you can use everyday in every relationship.