These are the books that I turn to again and again for inspiration and encouragement:
The Four Day Win
If I could pick JUST ONE, this would be it. It focuses on the transtheoretical model of change -- the stages that we go through when making any major change. You've probably heard them before... pre-contemplation, contemplation, preparation, action, and maintenance. This book's whole premise is that "diet books" focus on action. Sometimes they include maintenance. But 9 times out of 10 people fail in change because they haven't really finished "contemplation" and "preparation", and so "action" fails. So she leads you through each stage with exercises. The "four-day" part is because you set do-able goals, and do them for four days, and then reward yourself. It helps to set up a positive motion forward in your plan. It's not a diet. But the work you do in this book will help ANY diet you use. Get it from the library and see what I mean. I read half of it, and went out and bought my own copy!
You: On a Diet
This one is amazing for the science of how your body works, why you need to diet, and what happens when you diet. It explains things like cholesterol, hormones, artery damage, diabetes, inflammation, and blood pressure in ways that I can understand (and explain to others!). The second section also has a very solid diet plan, with some tasty recipes.
Thin For Life
This one is just downright inspirational. The author interviewed hundreds of people who have lost weight and kept it off, and analyzed the similarities in their methods. She's narrowed it down to 10 key things, written in a conversational manner and liberally laced with anecdotes from her research and discussions with people who have been there and done that.
Others that I like to pick up now and then, but aren't in my must-have list:
From Chunk to Hunk. Fred used to keep a blog online of his weight-loss journey, and he later got it published as a book. He lost a HUGE amount of weight, and kept it off, and this book takes you step by step.
The Flavor-point Diet. This one is an actual diet book. It's an interesting concept, though I'm not sure that it works. The premise is that if you lace the same flavor throughout your food during the day (mint one day, and lemon the next, etc.) you will be satisfied with less food. We tend to eat more when there is a variety of flavors. I'm not sure if that can fact can actually translate into eating less day by day, as in this plan, but I respect the author very much. I actually use this one for the recipes, more than the diet. The soups and salads are AMAZING. The eating plan is low calorie (around the 1500 range), and very tasty, so that by itself is worth getting it out of the library once in a while.