We'll let a Washington Post review of this book by Bunny Crumpacker introduce you to "The Omnivore's Dilemma":

Most of us are at a great distance from our food. I don't mean that we live "twelve miles from a lemon," as English wit Sydney Smith said about a home in Yorkshire. I mean that our food bears little resemblance to its natural substance. Hamburger never mooed; spaghetti grows on the pasta tree; baby carrots come from a pink and blue nursery. Still, we worry about our meals -- from calories to carbs, from heart-healthy to brain food. And we prefer our food to be "natural," as long as natural doesn't involve real.
In The Omnivore's Dilemma, Michael Pollan writes about how our food is grown -- what it is, in fact, that we are eating. The book is really three in one: The first section discusses industrial farming; the second, organic food, both as big business and on a relatively small farm; and the third, what it is like to hunt and gather food for oneself. And each section culminates in a meal -- a cheeseburger and fries from McDonald's; roast chicken, vegetables and a salad from Whole Foods; and grilled chicken, corn and a chocolate soufflé (made with fresh eggs) from a sustainable farm; and, finally, mushrooms and pork, foraged from the wild.

The first section is a wake-up call for anyone who has ever been hungry. In the United States, Pollan makes clear, we're mostly fed by two things: corn and oil. We may not sit down to bowls of yummy petroleum, but almost everything we eat has used enormous amounts of fossil fuels to get to our tables. Oil products are part of the fertilizers that feed plants, the pesticides that keep insects away from them, the fuels used by the trains and trucks that transport them across the country, and the packaging in which they're wrapped. We're addicted to oil, and we really like to eat.

Oil underlines Pollan's story about agribusiness, but corn is its focus. American cattle fatten on corn. Corn also feeds poultry, pigs and sheep, even farmed fish. But that's just the beginning. In addition to dairy products from corn-fed cows and eggs from corn-fed chickens, corn starch, corn oil and corn syrup make up key ingredients in prepared foods. High-fructose corn syrup sweetens everything from juice to toothpaste. Even the alcohol in beer is corn-based. Corn is in everything from frozen yogurt to ketchup, from mayonnaise and mustard to hot dogs and bologna, from salad dressings to vitamin pills. "Tell me what you eat," said the French gastronomist Anthelme Brillat-Savarin, "and I will tell you what you are." We're corn.

Each bushel of industrial corn grown, Pollan notes, uses the equivalent of up to a third of a gallon of oil. Some of the oil products evaporate and acidify rain; some seep into the water table; some wash into rivers, affecting drinking water and poisoning marine ecosystems. The industrial logic also means vast farms that grow only corn. When the price of corn drops, the solution, the farmer hopes, is to plant more corn for next year. The paradoxical result? While farmers earn less, there's an over-supply of cheap corn, and that means finding ever more ways to use it up.

[End of review excerpt]

This is NOT a vegetarian or vegan book (although you may become one after reading it). It is as the title indicates a personal reflection and study of the the relationship between the human species and food. In that context, omnivore is defined as "an animal that will feed on any kind of food, including both plants and animals."