Sidewise Award for Alternate History - Long Form (Novels)
The Sidewise Award for Alternate History was established in 1995 to recognize the best alternate history stories and novels of the year. The award is given annually for Short Form fiction and Long Form Fiction, including novels.
There was a tie in 2002, and the winner in 2006 was a trilogy.
Edit: 9/12/13 to add 2010-2012