First, assuming you're talking about cooking (rather than baking)...
I think cooks fall into two categories - the recipe followers, and the improvisational cooks. Neither is necessariliy better than the others - improvisationals (which is where I put myself) like the creative and experimental process, although we have to study certain recipes for a rough idea of what a certain process is, and then the recipe cooks that are always looking for good recipes - and there's certainly nothing wrong with that, as there's a zillion good recipes out there - the only problem (in my mind) is finding the best ones, but I guess that's the attraction if you mainly work from recipes.
So, anyway, as said, I land more on the improvisational section. When I moved into my current house I took the opportunity to put on the shelf the few books I find invaluable (or interesting), the others are around for reference but they're not the ones I run to first.
My collection of must-haves (for me, from the improvisational point of view) are heavily slanted toward theory and technique. The list is (in no particular order):
In The Kitchen with Rosie - Rosie Daley (don't ask me why, but somehow it persists on being on my shelf)
The Ethnic Food Lover's Companion - Eve Zibart
On Food and Cooking - Harold McGee
Roasting - Barbara Kafka
Savoring Herbs and Spices - Julie Sahni
The French Laundry Cookbook - Thomas Keller
Think Like a Chef - Tom Collicchio (this is probably my favorite)
Craft of Cooking - Tom Collicchio
Sauces - Classic and Contemporary Sauce Making - Peterson
Cookwise - Shirley O. Corriher
The Way to Cook - Julia Child
Professional Cooking - Gisslen
How to Read a French Fry - Russ Parsons
The Young Man and the Sea - Pasternak and Levine
An American Place - Larry Forgione
Biba's Italian Kitchen - Biba Caggiano (my wife's family is Italian, so this has helped)
The New England Clam Shack Cookbook - Dojnay (I don't know how it ended up here, but there's always something)
Not on my shelf, but I know I have it - the Frugal Gourmet on our American Immigrant Ancestors or something like that. Very good.
As mentioned, I like Tom's book the best (and he'll talk to you too, or email at least), On Food and Cooking (McGee) and How to Read a French Fry (Parsons) are more scientific theory - Parsons is more accessible - half the time I think I need a chemistry degree to get through McGee, but he is the ultimate reference for food science.
The Young Man and the Sea is a very interesting cookbook - I'd suggest at least checking it out; it's a unique style of fish cooking and I confess not having spent enough time working on it. I really want to.
I have some others that are specific to certain ventures (like pizza making) that I won't list here, and I think a subscription to Cook's Illustrated is invaluable. They are kind of like Consumer Reports in that they don't accept advertising, and they go off on ventures like "How to cook the perfect (whatever)" and will go through 50 turkeys or whatever in the experimental process, so you gotta like that. They're not always perfect; I disagree with some stuff, but overall they're constantly worth checking out, if only for a starting point.
The Ethnic Lover's Food Companion is worth checking out; it has a lot of interesting style and technique information on various ethnic cuisines, and how to approach them in America (meaning, with access to spices and stuff available in this country, rather than some obscure thing you'd never be able to find or order).
The French Laundry Cookbook comes off as a coffee-table book, but Thomas Keller is one hell of a chef, and it's interesting to study some of his unique ways of doing things.
Anyway, these generally form my starting point.
Lest I forget, though, I also have to recommend a book that I got for a friend of mine from the south, who was a professional chef in New Orleans, and actually despite the format of the book, it gives some very authentic southern food recipes. It's not a genre I usually chase down, but despite the title it's actually a decent book if you're into southern food, and that would be "White Trash Cooking" by Ernest Mickler:
http://ecx.images-amazon.com/images/I/51S98NTKSML._BO2,204,203,200_PIsitb-sticker-arrow-click,TopRight,35,-76_AA240_SH20_OU01_.jpgEnjoy.