Yes, it was based on the book but they changed certain plot elements.
I have finished watching it meanwhile and have to agree that it wasn't a particularly good adaptation in my view. Dominic Cooper as Henry and Rufus Sewell as evil Adam did a great job, and I also loved the actress who played Lincoln's wife (can't remember her name for the life of me right now!), but to me, the movie wasn't very well structured. It seemed like one scene slapped on another without much to connect it. Yeah, right, it was all about Old Abe fighting vampires, but somehow I missed the thread holding it all together. Too many vampires popping out from somewhere, snarling and baring their fangs, too little storyline.
Another case of a book that I liked so much more than its adaptation.