We've all felt the giddy flutter of excitement when our new lover walks into the room. Waited by the phone, changed our plans...But are we in love, or is there something darker at work? In ''Desire: Where Sex Meets Addiction,'' Susan Cheever explores the shifting boundaries between the feelings of passion and addiction, desire and need, and she raises provocative and important questions about who we love and why. Elegantly written and thoughtfully composed, Cheever's book combines unsparing and intimate memoir, interviews and stories, hard science and psychology to explore the difference between falling in love and falling prey to an addiction. Part one defines what addiction is and how it works -- the obsession, the betrayals, the broken promises to oneself and others. Part two explores the possible causes of addiction -- is it nature or nurture, a permanent condition or a temporary derangement? Part three considers what we can do about it, including a provocative suggestion about how we describe and treat addiction, and a look at the importance of community and storytelling. In the end, there are no easy answers. ''A straight look about some crooked feelings,'' Desire shows us the difference between the addiction that cripples our emotions, and healthy, empowering love that enhances our lives.