Peggy Carlson, MD has compiled the ultimate source on vegetarian nutrition. Check out The Complete Vegetarian: The Essential Guide to Good Health (University of Illinois Press, 2008). Each chapter provides a comprehensive analysis and review on topics most important to vegetarians, including all of the relevant nutrients, common diseases that are affected by diet, vegetarianism in childhood, pregnancy, and lactation, sports nutrition for vegetarians, and vegetarian meal planning.
The chapters are written by experts in their respective fields and reviewed by Dr. Carlson. Unlike your basic "for dummies" books on vegetarianism, this one is "for smarties" -- it provides the latest scientific research and information, and puts it into a context that we can all relate to and learn from.
One particular area, in my opinion, that seems to swim in controversy is fats in the vegetarian diet. How much omega-3 do we need? How are the needs for fats different for vegans compared to vegetarians and omnivores? Should we take supplements? These questions and more are answered in the chapter on fats, written by Brenda Davis, RD, leading expert on fatty acid nutrition.
Dr. Carlson herself wrote most of the chapters on chronic diseases, and they provide a really in-depth review of the role of diet in preventing, managing, and reversing these conditions.
You can buy it from amazon or other booksellers.