On the whole an interesting work, but not without its weaknesses. First, Forrest gets short shrift vs. Grant in terms of the narrative, second I don't think Hurst proves that the Pillow/Donelson battles "decided" the Civil War, and finally, I personally had a hard time following his blow-by-blow description of the battles. That last maybe is a failing of mine, but oh, well.
I'm also a bit skeptical that Halleck was quite the bucket of scum Hurst paints him as. Not that he wasn't grossly unfair to Grant, of course, but Hurst really goes to town on him. This is the first place I've ever read allegations that Halleck essentially made up a story in hopes of relieving Grant...and I've read quite a few Civil War books in my time.
Still, kind of an interesting hypothesis underlies the book, one I'd never considered. If I'm not quite convinced, I must admit to being at least a bit intrigued.