This started as a comment to a diary asking about vegetarian nutrition, Tips for becoming a vegetarian?, by bluefaction.
There are lots of resources for vegetarian cooking and nutrition, especially on the Net. The one cookbook I would not want to be without is Laurel's Kitchen, which includes a good grounding in all the essentials of vegetarian nutrition. For a while, especially when I was Chief Cook in a Buddhist monastery, I collected vegetarian cookbooks, and of course I adapted many meat recipes to use with meat substitutes. There are excellent meatless dishes from Catholic countries (for Lent), and from Buddhist and Hindu countries, and whole cookbooks about that side of their cuisines. Peasant cooking has a lot to offer, whether from Latin America, Eastern Europe, Asia, or elsewhere.
I'm not going to belabor what you can easily look up on the Internet. Just a few essential points about vegetarian nutrition, and then some suggestions off the beaten path.
Everybody turning vegetarian worries about protein. It turns out that traditional peasant diets have dealt with this. Combining rice, beans, and tortillas with a bit of cheese makes an excellent balanced protein source. Potatoes and milk, as in Old Ireland, or cereal with milk. Bread with bean, pea, or lentil soup. Venetian pasta fazool (pasta e fagioli, pasta with beans). Rice and tofu. Rice, dal (lentils and other legumes), and any of the amazing variety of Indian breads, including pappadam, chapati, paratha, naan, and more. For the techno-peasant, peanut butter sandwiches.
Americans eat way too much protein, though not anything like as much as the British aristocracy used to. That was one of the chief reasons for the prevalence of gout in the 19th century. We also get too much fat and carbohydrate, and not enough fruits and vegetables. Don't try to force yourself. Find stuff you actually like. (Mm, pluots. Blueberries in sour cream. Spanakopita. Eggplant Parmesan. Home-grown tomatoes. Minestrone with pesto and Parmesan. Toad in the hole (Yorkshire pudding with sausages baked in) with yeast gravy. Apple pie with a slice of cheddar. YMMV.) British and American cooking are particularly harsh on vegetables. Nobody else that I know of believes in boiling them to death without any flavorings beyond butter and salt.
Vegans have to be careful about vitamin B12, which occurs in very few non-animal sources, but is added to various foods today. Lack of B12 for a number of years leads to a nasty disorder called pernicious anemia. Look it up, make a few notes, pay attention, and it won't be a problem.
Sausage and fried chicken are the only things I ever missed. I can recommend Tofurkey Italian, Smart Sausage Chorizo, and Field Roast Chipotle, but I wouldn't insist. There are dozens of others. Worthington, Morningstar, and other brands have adequate chicken and hamburger replacements. Our family favorite is Morningstar Original Chik Patties. Any Chinese grocery should have a freezer section and a canned goods section for imitation meat and seafood products such as mock fish, mock duck, mock chicken, mock abalone, and mock popk. (That's what it says on the can.) Pick up a variety of fresh or canned mushrooms to go with them, and explore the variety of Chinese vegetables.
In addition to tofu, I like tempeh (Indonesian-style fermented soybeans) in a stir-fry with coriander powder, garlic, and soy sauce, and various veggies.
How about fishless sushi? Traditional varieties include
Tamago, egg
Kappa, cucumber
Inari, soy skins
Natto, fermented soybeans (not for everyone)
Kampyo, squash
Ume-shiso, my favorite, pickled plum and beefsteak leaf
Most sushi restaurants have all but the last of these on the menu, and many will make umeshisomakizushi as a special order if they have the ingredients on hand. You will impress the sushiya-san if you order it. (More if you actually like it, of course.) You can make your own rolled sushi, and other kinds, with all sorts of ingredients. Look them up.
Then there is kimchi, on which opinion outside of Korea is strongly divided. To some, like me, it is ambrosia, while to others, like my son, it is toxic sludge. (I was in the Peace Corps in South Korea, starting in 1967.) Just check the label to see whether shrimp or anchovies are among the ingredients. And be aware that in Korea there are dozens of kinds of kimchi, just as Heinz used to be known for 57 kinds of pickle.
A slang term for half-Korean, half-Mexican children is "kimchi burrito", in part because they actually eat such things. I've tried them (the food) and they are not bad. I don't think they will catch on, though.
For those who do like kimchi, I can recommend kimchi-cchigae, a tofu and kimchi stew available in most Korean sun-tubu (fresh tofu) restaurants. Beoseossuntubu (tubu and mixed mushrooms) is also excellent. Along with your rice, soup, and kimchi, you will get Korean namul, lightly cooked veggies seasoned with sesame and garlic. Korean spinach namul is the best thing that ever happened to spinach.