I just wanted to suggest some great books:

Eating Animals by Jonathon Safran Foer
Why We Love Dogs, Eat Pigs and Wear Cows by Melanie Joy
The China Study by T. Colin Campbell- excellent book about nutrition based on the largest nutrition study ever conducted.

Foer write his book after having his first child and feeling compelled to find out more about the source of his family's food. Joy explores interesting ethical issues in her book. Specifically how we compartmentalise how we regard animals. We would never consider harming or eating our companion animals, but we turn a blind eye to the suffering of animals like pigs, who are as intelligent as dogs.

Regarding B12, Meow is correct. If you are still eating dairy and eggs you generally wouldn't have to worry about taking a B12 supplement. B12 is actually a bacteria, not an animal product. It is present in the guts of animals, including humans. When people consume animals or animal products they also consume B12.