My favorite, hands down, and the one I always recommend, is The Low Carb Cookbook, by Fran McCullough. Everything I've tried from this book has been wonderful. I especially recommend the Gratin of Turnips! You think I'm kidding, but when my sister served this recipe at last year's Thanksgiving Dinner, her brother-in-law told her it was the best "potatoes au gratin" he'd ever had! A must for every low carb kitchen.
There are also two low carb cookbooks from Dr. Atkins. I have them both! Many good recipes, and good ideas. There's the Dr. Atkins' New Diet Cookbook -- this is the bigger book. The Tandoori Chicken recipe from this book is one of the best dishes I've ever made in my life, low carb or not -- keeping in mind that I love spicy, exotic food. (On the other hand, I tried the Gnocchi recipe, and even the dog wouldn't eat it!) An excellent resource.
Dr. Atkins' Quick and Easy Cookbook -- I really liked the Hazelnut Torte from this one. Some recipes call for you to buy Atkins' Bake Mix and such. Worth buying!
I have to include Neanderthin in this list! Ray Audette's book on why we should eat as closely as possible to the diet our caveman ancestors ate is just too cool to miss! This is one of the most fascinating books I've read on the subject of diet.
There's another cookbook I just have to recommend, although it's not a low carb cookbook. It's The Complete I Hate To Cookbook, by Peg Bracken. No, not all the recipes will work for a low carb diet, but I've found many recipes in here that either will work for us, or are easily adapted. Better yet, they're all reasonably easy to make, and tasty! I turn to this book often when I need ideas. Furthermore, Peg Bracken is a wonderful writer, and this is the funniest cookbook I've ever read. Highly recommended.
If you don't have a good food count book -- a must! -- here's one I rely on: Corrine Netzer's The Complete Book of Food Counts.
If you'd like listings for more than calories, fat, carbs, fiber, and protein, there's also Bowes and Church's Food Values of Portions Commonly Used. Very, very helpful if you're looking for a low carb source of a particular nutrient, whether it's calcium, potassium, magnesium, or whatever. I turn often to this book.
I think Protein Power is a wonderful book. It does have a LOT of medical jargon, but it also has a nice, simple summary at the end of every chapter if you want to cut to the chase. Essentially similar to Atkins, but more fiber, more fruits and vegetables!
Another good book by the Hellers is Healthy For Life, which is more about controlling your carbs to improve your health, rather than to lose weight. It's one of the least radical programs, and good if you're really scared of low carb, or if you have family member or a doctor who needs convincing!
The Schwarzbein Principle is gaining a lot of followers, but it's a newer book and I haven't read it yet. I do know some folks who've had good results with it.
Sugar Busters is more about cutting out sugar and the other worst carbs, like white flour, rather than a strict low carb diet. Worth reading, and not all full of jargon.