While it lacks the encyclopedic breadth of Joy of Cooking, the idiosyncratic personality of celebrity chef cookbooks (Emeril's kick-it-up-a-notch joviality, Rachael Ray's Minnesota soccer mom practicality, etc.), and the luscious photos of "food porn" cookbooks (like the Williams-Sonoma series), The New Best Recipe by the editors of Cook's Illustrated Magazine is by far the most useful cookbook I've seen.
These guys--who also produce the nerdy PBS show America's Test Kitchen--try hundreds of variations of each recipe, and tell you what works the best. Better yet, they tell you why it works the best, so you can make informed decisions about how closely to follow their recipes. They don't get super-technical, like Alton Brown's Bill-Nye-the-science-guy wackiness, but they explain each of the experiments they tried and how they arrived on what they think is the "best" recipe. Furthermore, they explain how to cut out time and effort without cutting out taste. Also, like the cooking show and magazine, they include product reviews and taste tests, which will help you make informed supermarket decisions. Yes, there are plenty of other good cookbooks out there, but until proven otherwise, I'm convinced this one is the best.
(The one downside: as Kathy Grace mentions in her otherwise glowing Amazon review, it has a deplorably bad Table of Contents. The Index isn't much better.)
Update: a website that includes reviews, recipes, and kitchen tests with a similar spirit is Cooking for Engineers--a great resource. You don't need an engineering degree to understand it, but those who think like an engineer will like it.
Update #2: I am hearing good things about Mark Bittman's How to Cook Everything...perhaps it will give the Cook's Illustrated folks a run for their money.... Also, it seems that everyone's favorite "cooking science" book is Shirley Corriher's Cookwise.