I did read the book, which was really interesting but it would have made a grim movie translated directly. The film retools the story and changes a lot, making the story into a "fun ride" instead of a "grim ride."
The whole thing is played with a straight face. It's the best action/horror flick since "Sleepy Hollow," I think, and just because the humor is handled so subtly, I feel it is much more effective than "Army of Darkness." AOD is funny, of course, but so goofball that the viewer never gets out of the crude 3 Stooges mentality. "Abraham Lincoln" features jokes that are subtle, refined and sophisticated.
It also helps to know a little bit about the actual Abraham Lincoln's real life, as well as the Civil War and the general political climate of that time...the viewer is rewarded for being somewhat literate.
Again, I cannot recommend this more highly. Here's my imdb review:
http://www.imdb.com/title/tt1611224/reviews-100