I love anything by Haim Ginott, especially "Between Parent and Child". It's not so much about discipline (as in how to get kids to do what you want, gently) but a way of communicating with your kids. I believe that the side effect of following Ginott's advice will be a better relationship with your kids and that can't help but manifest itself in "discipline issues."
I love "Kids, Parents and Power Struggles" by Mary Sheedy Kurcinka because it helps us to understand where some of the resistance comes from when we ask our kids to do things, or want them to behave.
Alfie Kohn's "Unconditional Parenting" is ANYTHING but a how-to. But it's good for shaking up your thinking and looking at things in a new way. Then Kurcinka's book will help you put stuff into actual practice.
I also like "How to Talk So Kids Will Listen and Listen so Kids Will Talk". I am not all the way through it but it's been very helpful so far.
These aren't "discipline" books per se, but I think that they are parenting books that will help foster a gentler more respectful environment and relationship, which helps in the area of discipline.