I like Adventures in Gentle Discipline by Hilary Flower. It's the kind of book you can open up to any page and find something useful. You don't have to read it in order, so it is the perfect, bathroom/bedside table book. Also, for any particular issue, it has an array of GD perspectives, which I like.
It has a very positive tone, toward both children and parents. I try to read from it a few minutes a day, and I think it really helps keep me in the kind of place I want to be with my kids.
A very different book I really like is Parenting from the Inside Out. I think in many cases people know the right way to care for children (they want to use respecftul, nurturing, nonviolent methods), and when they don't follow that route it is often because their emotions have gotten in the way. This book helps you figure out how to be more self aware so you can parent the way your wiser mind knows is best.