When I read the description of "Darker Angels" I was excited - I love alternate histories and fantasy books involving historical figures, a la J. Gregory Keyes or Tim Powers. The cast list was impressive - Abraham Lincoln, Walt Whitman, Poe, Marie Laveau, George Gordon (Lord Byron). But this shambling, soulless mess of a book didn't even live up to the Kirkus Review.
It is perhaps appropriate that a novel about zombies in antebellum America be as lifeless and dead as this one. But seeing the aptness is not the same as being entertained or enlightened. Somtow's primary gimmick - having a character within the story told by the narrator tell a long story, in which another story is told by another character, in which another story is told by a character in THAT story - quickly becomes tedious and confusing. If you like that sort of thing read Melville, who can pull it off with artistry and subtlety. Somotow just doesn't have the chops.
Add to all this Somtow's preoccupation with male anatomy and necrophilia and you have a book about as appealing as poodle lasagne.
I'd suggest you give it a miss, or wait until you stumble across it in a used bookstore.