From the review over at the American Presidents Blog:
In this alternate history, Lincoln becomes at an early age a vampire hunter and this shapes his entire life including his later political career. The American frontier is full of vampires up to no good and as Lincoln travels he moves further up the ranks of those fighting the vampire menace.
Eventually, Lincoln learns that slavery is an institution that vampires use to keep an easy source of human victims. The entire southern system is based on patronage from vampires. The decision to run for president and the ensuing American Civil War follow from this fact. I don't want to give anything away but I did not see the ending coming and it was very satisfying. (Did you know that John Wilkes Booth was a vampire?)
Seth Grahame-Smith is a good author and he paces the book well. His writing is interesting and it was hard to put the book down until I finished.