Blood Oath: The President's Vampire Nathaniel Cade Series, Book 1.

In 1867, two men went missing on a whaling vessel out of Boston Harbor. According to published reports, the ship's captain found another sailor standing over their corpses, drinking their blood. Tried for murder, the sailor was pardoned by President Andrew Johnson and died in an asylum for the criminally insane. At least, that's the cover story. In fact, nineteen-year-old sailor Nathaniel Cade was turned into a vampire. His life was only spare...