Although Canada (British North America) was considered neutral in the American Civil War (1860-1865), with between 30,000 to 50,000 Canadians fighting for the Union (Northern), and the Confederates (Southern) side of the conflict.
They were signed-on by the recruiters, and according to a book by John Herd Thompson and Stephen J. Randall called Canada and the United States, Canada refused to return 15,000 American deserters, and draft dodgers.
Four of the sites which may interest you are -
http://pvtchurch.tripod.com This site is very comprehensive. It gives the complete history of the Canadians in the American Civil War, has letters written by the soldiers back to their homes, and the history of The Underground Railway, the way taken by the slaves to come to Canada.
www.suite101.com/article.cfm/canadian_tourism/97383 Gives a brief history of Canadians in the American Civil War.
www.rootsweb.ancestry.com/~canmil/cw/index.html This site is maintaied by Brian Massey, the great-grandson of William Massey who enlisted in the war Tennessee, although he was from the provience of Ontario.
www.ancestry.com/civilwar_sub?o_iid=47474&o_lid=47474 Ancestry.com has updated it's holdings, and is offering a 14-day free access to the database.