These are, for me, the absolute essentials - my go-to guides, the books with torn, stained pages and dogeared corners.

The Joy of Cooking, by Irma S. Rombauer, Marion Rombauer Becker, and Ethan Becker
You might be surprised that somebody who almost never cooks with meat still uses The Joy of Cooking as a primary handbook.  However, I continue to find it immensely useful.  As a longtime vegetarian I became expert at adapting recipes, and this book provides most of the base recipes I need, along with all of the handy conversion charts and tables that an experimenter will need to make use of regularly! Sure, I'm never going to use the recipes for corn dogs or beef fondue, but I find myself cracking this book open at least once a week.  If I find myself asked to cook in a kitchen without it, I miss it tremendously!

Passionate Vegetarian, by Crescent Dragonwagon.
Despite the rather offputting authorial moniker, this has become my absolute favorite cookbook.  I've never made a recipe I haven't absolutely loved.  I take out this book for slow-cooking days, when I want to spend a few hours relaxing in the kitchen.  I love the many variations listed for most recipes, and the tips for using leftovers or for serving at parties.

The Sunlight Cafe, by Mollie Katzen.
It is almost sacrilegious for someone to include books by Mollie Katzen, but not include The Moosewood Cookbook or The Enchanted Broccoli Forest. Yet, this book gets the most use of the many Moosewood books and Mollie Katzen books on my shelf. Partly because I often take time to cook brunch on Sunday, when I'm most relaxed, and partly because the recipes provide easy tips for even healthier versions using soy or whole wheat flour, this book comes out to play at least twice a month.