I strongly dislike most celebrity chef cookbooks. In many (not all) cases, it's meaningless branding that doesn't guarantee a minimum level of quality or a particular cooking style.

One of my bibles is Le Cordon Bleu Complete Cooking Techniques. I also gave this to my sister and her husband at Christmas, and they've loved it. It's something to sit in bed and read, and then try things out of. You'll discover things you never even thought of cooking (my sister didn't even know you could make homemade mayo, now she absolutely loves the stuff).
I shop at farmers markets a lot, and frequently come across veg I'm not familiar with, or new cultivars of familiar veg. Vegetables: Amaranth to Zucchini is excellent for giving guidance on how to cook with them.
If you have access to offal and game, then Nose to Tail is invaluable.


My father has also enjoyed "License to Grill", so that comes recommended too.

Not cookbooks, but outstanding books about food, culture and biology:
Catching Fire: How Cooking Made Us Human
In Defense of Food, Michael Pollan's most accessible (and most interesting) work.
Also worth reading is Taste: The History of Britain Through Its Cooking for an interesting historical look at the way that tastes have changed over 2000 years.