I basically have put a moratorium on new cookbooks. I just don't need anymore. I'll make an occasional exception (my stepdaughters have each given me EXCELLENT cookbooks I didn't ask for: Mark Bittman's How to Cook Everything Vegetarian and Alice Waters' Chez Panisse Vegetables) but mostly I just stick to what I have and they all have plenty to teach me.
My two most instructive books are Mark Bittman's How to Cook Everything (that includes meat) and Julia Child's The Way to Cook (which, rather than Mastering the Art of French Cooking, is my Julia bible). If I'm encountering a new cut of meat or some other new element in my cooking, I always consult both of them ("What would Mark and Julia do?") to understand the principles of the challenge in front of me.
But the cookbook that I turn to over and over again that has never failed me -- especially for dinner parties -- is Patricia Wells' Bistro Cookbook. (By the way, all of these books are available in the Urbal Tea Store.)
I was reminded of how much I love this book last night when I made her tarte tartin aux poires (a tart tartin with pears instead of apples) for my stepdaughter and her 16-month old who came to dinner last night. It was an amazing tart, I must say, and even though Paloma was very busy bouncing balls and running back and forth, she still had the wherewithal to return to her mother for "more" bites of tart.
Wells gives you really great guidance (for instance, on how to get the pears nice and brown and carmelized without burning them) and she makes things easier rather than harder (for instance, she has you pour the pears into a casserole for baking in the oven with the crust, rather than in the cast iron pan, which is so heavy to flip over when the time comes).
I have never done a recipe in that book that didn't work, which I think is highest praise for a cookbook: her leg of lamb roasted over a gratin of vegetables (I've modified the recipe so that it's root vegetables) is the surest dinner party winner I've made; her golden cream and apple tart is the best apple pie I've ever had; the pissaldiere comes out perfect each time; and her recipe for Madame Cartet's potato gratin is as good as you'll get if you go to the bistro of the same name in Paris (which I did and, while being in Paris definitely improved the dish, Wells' recipe is astonishingly true to the original).
My dearest friend from high school, Barb, who is the best entertainer I know, swears by Wells' creme brulée, which I haven't made (yet). Barb, in many ways, was my first inspiration for wanting to cook. I recall being at a party she was giving, and she looked at people devouring the offerings on her buffet table and said to me, "God, I get such pleasure out of watching people eat my food." I caught the cooking bug, I think, at that moment. And when I make something from Bistro Cooking, I know exactly what Barb meant.
P.S. I think the other best cookbook I own came from Barb, years ago: Marian Morash's The Victory Garden Cookbook. It is my bible on vegetables: how to freeze or otherwise preserve them; understanding yields; basic treatments for them (for instance, how many minutes to steam a green bean vs. a lima bean). If you have an urban or suburban vegetable garden, this book is indispensable.