I'll start with the wonderful and out-of-print The Yogi Cookbook by Yogi Vithaldas & Susan Roberts. Philosophy and cooking.
Diet for a Small Planet by Frances Moore Lappe was important to me in the late 60s.
The Vegetarian Epicure by Anna Thomas - and she was a filmmaker too!
Keep it Simple by Marian Burros - great for working parents with kids.
The New York Times Cookbook by Craig Claiborne. Short but great recipes.
The FarmHouse Cookbook by Susan Herrmann Loomis. Delicious and the ultimate terrific research -- going to farms and eating.