As a personal trainer and a person who has had a lifelong interest in nutrition (I began reading nutrition labels at a very young age and read my mom's college nutrition textbook cover to cover in middle school!) I have been through my share of diet books. Not much impresses me these days.
The first two books listed here are my personal favorites for diet books that aren't really diet books at all...rather, they teach you how to choose healthier foods that you enjoy that lead to natural weight loss. These books help you find a new way of eating that doesn't feel like a diet--which means you can stick to it for life, and let the pounds fall away easily without feeling hungry or deprived, and actually improve your health in the process.
This is my book, which solves one of the major problems inherent in most diet books and programs--lack of follow-through. Once people read about and learn a diet program or nutrition tips and techniques, they often fail to implement their newfound knowledge into everyday life. It's not about discipline, it's about changing your mindset. "Mind Over Fatter" helps you gain the mental strength and focus to make important changes in your life to allow you to be happier and healthier.
My other favorite fitness and nutrition related books are:
--Starting Strength, by Mark Rippetoe and Lon Kilgore
--The Maker's Diet, by Jordan Rubin