The Omnivore's Dilemma, and loved it so much that I decided to try In Defense of Food.
I didn't like this book as much. It was well-written--Pollan has a very nice style--and had an interesting perspective on how one should eat. But I don't usually like diet books, and this is basically a diet book. They tend to be simplistic in their thinking and use a lot of shady science.
To Pollan's credit, he notes that very little is actually known about nutrition and food's effects on the human body. But then he goes ahead and makes recommendations anyway. For example, he notes that organic or heritage food has a different chemical composition than the more conventional varieties. This is very interesting and thought-provoking. But then he assumes that these differences mean organic food is superior (even though elsewhere he's stated that we don't know exactly what these differences consist of, let alone what they mean). It very well might be superior, but maybe it's also full of various toxins (since organic plants have to fend off pests); the state of current information is just not good enough to know. Nature is not a benevolent force after all (See: brain-eating bacteria in the water).
Most of his recommendations made intuitive sense, like that eating vegetables is very good for you, or lots of corn syrup is probably not a good idea. However, the glaring omission in his "diet rules" was the role of total calories or portion size. There is a lot of evidence that 1. calorie restriction prolongs life and 2. being fat is really, really bad for you. Americans have gotten a lot fatter in recent decades, and while there are many reasons for this, the main one is just that we eat too much. (The average American consumes 500 more calories than he did in 1970.) Every time I return to the US, I am shocked by the enormous portion sizes and constant snacking.
Pollan does say eat "not too much". But the book's focus is primarily on things like eating more vegetables, local produce, organic/free range meats and eggs, and unprocessed food. Since the book's target audience is American, if health is his goal he needs to focus heavily on portion size and eating less. The average American already eats 20% more fruits and vegetables than in 1970, so stressing an increase in these is not that helpful.
These criticisms aside, the book was well-written and the best diet book I have read (though admittedly this is a very low quality genre). And most of Pollan's recommendations were sensible and well-put. Following his advice might not make you healthier, but it wouldn't hurt you either.