I'm not much of a cook, except for a short list of special dishes I've had enough practice with to do right. Most of my time reading cooking books has been goggling at the more obscure recipes in The Joy of Cooking (walnut ketchup?!) That's my favorite cookbook, though, because it doesn't assume you already know things. You can find an explanation of how to do or how to make whatever in that book, it seems. But it isn't so much a reading-straight-through book.
What Einstein Told His Cook: Kitchen Science Explained by Robert L. Wolke (with recipes by his wife Marlene Parrish) can work either as a reading-straight-through or a look-up-one-thing book. It's definitely a cookbook second, through, and an everyday science book first. You can learn about how the sense of taste works, the different types of "raw" sugar, the FDA labels for different types of cocoa powder, the difference between the cooking definition of salt and the chemical use of that work, or what the difference is between fats and fatty acids. And that's just in the first three chapters. If you want to know how your water filter works, what MSG really is and why some people avoid foods containing it, what grits are (and even the U.S. Southerners who eat them don't necessarily know how they're made), the five processes used to cure hams, and some rather gross stuff (in my humble opinion) about raw shellfish, this book has it all in one neat package. Perhaps I'll remember to add an additional comment to this entry once I've actually tried the recipe for Mocha Soy Pudding.
(And checking Amazon.com while posting this entry, I find that there's a second volume now. I feel confident that I can recommend that one too, despite not having known it existed until a few minutes ago.)