For information about OCD and related disorders, I highly recommend the International OCD Foundation (IOCDF). Their website and conferences are excellent; the IOCDF Newsletter, free with membership, is strongly recommended. Membership in IOCDF is a very good value and highly recommended.
The IOCDF website lists many excellent books on OCD and OCD spectrum disorders. You can support the IOCDF by going there first, and choosing a book from their list -- this will take you to Amazon.com to fulfill your order.
I recommend that everyone undertaking cognitive-behavioral therapy (CBT/ERP) for OCD strive to become knowledgeable about the therapy, since most of the actual work of therapy is done by the person with OCD, outside the therapist's office, in daily practice. Family members of people with OCD need to learn how to be CBT/ERP therapy aides or coaches, as they will be "on the front lines," helping their loved one to respond appropriately as events occur in the person's life. Family members and significant others also need to learn not to make the common mistake of reassuring the person with OCD. This is not as easy as it sounds!
One book I happen to think highly of is the following:
You are not the only person interested in learning more about obsessive-compulsive behavior. The most severe form, Obsessive-Compulsive Disorder (OCD), is the fourth most common psychological problem in the United States of America, affecting one out of every 30 to 40 adults, and one out of every 70 to 80 children. It's hard to gather statistics on the milder cases of obsessive-compulsive behavior, which are more common but often undiagnosed, as a mild form doesn't significantly interfere with a person's life. Probably everyone has some degree of obsessive-compulsive behavior. We all have our quirks that make us us. The vast majority of obsessive-compulsive behavior does not need to be treated.
In this book, you'll learn plain facts about the physical causes and emotional experiences of obsessive-compulsive behavior, and the treatments which are effective at improving the lives of people with this condition. Symptoms of obsessive-compulsive behavior often increase and decrease over a lifetime, and can change from one type such as washing to another such as hoarding. People with even the severe form are often successful at hiding their obsessions and compulsions from others, and suffer with it for years before seeking treatment. A complete cure for obsessive-compulsive behavior does not yet exist, but treatments to help people control it are very effective, so that it becomes more of a minor annoyance than a major problem.
If you're looking to understand obsessive-compulsive behavior in yourself or someone you care about, and to find ways to cope with it and improve it, this book can get you started.