This is an elusive vitamin not produced by animals or plants but by simple bacteria. It is found in animal products but rarely in any vegetable sources. B12 deficiency is rare but quite catastrophic. Yeast by-products provide a source. There is considerable debate about the ability of humans to synthesise their own B12 through bacteria in the digestive system as some vegan cultures appear to survive without obvious intake of the vitamin.
A further concise article with some good advice for Vegan nutrition.
I would recommend Sanatogen Vegetarian as a supplement to a vegan diet. It contains the necessary B, B12, and D vitamins as well as calcium & iron. I'm not sure that supplements are necessary for a Vegan diet but I would always recommend a 'just in case' approach! This way you will probably be far better off than the average omnivore. Iodine and Omega-3 fatty acids yet to be investigated.
Definitely a recommended site for putting things into perspective. This site is especially critical of the more extreme natural hygeine/fruitarian diets, and highlights the possible problems with veganism that some people try to ignore in order to push their latest fad diets onto the gullible.
These books are more useful for Vegans/those wishing to explore the variety of Vegan cuisine:
Here also are some books covering the "animal rights" issue...
Work in progress on some factory farming/vivisection info pages.