In Defense of Food: An Eater's Manifesto by Michael Pollan
The cover of In Defense of Food advises "Eat Food. Not Too Much. Mostly Plants." Small parts of it are recycled from The Omnivore's Dilemma, which is an amazing and wonderful book, but there's also a fair amount of original stuff about nutrition research that was pretty eye-opening. His conclusions about how to eat for health weren't earth-shattering to me, but they were useful and are helping me get back on track after feeling kind of derailed about what we're supposed to be eating. I liked it a lot.