If you're interested in the history of U.S. vegetarianism/veganism, you might want to start with Sylvester Graham (1794-1851) and the early American health food movements he inspired. Graham instructed his followers to abstain from animal products and to eat less in general among other habits related to simplicity and moderation. Graham's teachings on food seemed to have been based on a mixture of religious and health considerations, with particular concern paid to curbing sexual urges. He does not seem to have been concerned with animal welfare. He is mostly remembered as the inventor of the Graham cracker, which now bears little resemblance to its original form.

Though the Grahamite movement was relatively short-lived, Graham inspired John Harvey Kellogg (1852-1943), who went on to influence the Seventh Day Adventists' teachings on food and to found the Battle Creek Sanitarium, a health resort that subjected patients to various dietary and physical treatments. Kellogg's vegetarianism and food advocacy was more health-focused than religious, and non-Adventists often visited as a "cure" for the ill health and poor digestion thought to be brought on by city living. The fame of the Sanitarium sparked increased public interest in vegetarianism as a health remedy. As with Graham, Kellogg is primarily remembered for the products sold under his name today. (Well, not exactly his name. It was his brother Will who commercialized and popularized breakfast cereal, mostly by adding sugar to John's formula, to which John strongly objected.)

For further reading, Vegetarian America, though it has a decidedly pro-vegetarian slant, is filled with interesting excerpts from contemporary sources on Graham and later vegetarian movements. Also, Graham's book on breadmaking is still in print, as are many of his writings and speeches. You might also want to check out T.C. Boyle's novel The Road to Wellville. Like most historical novels, it's heavily fictionalized, but well-researched and an enjoyable read.