Important book, but disappointing, especially given the hype around it.
Positives: Helps make sense of the slash-and-burn politics of the GWB administration by showing how they trace to strategies (whisper campaigns, Big Lies, ab/use of prosecution power to advance political goals) first honed by Rove in Texas state politics. Even at this late date, I found fresh reason for outrage and dismay. Also, the journalistic style makes it a quick and enjoyable read.
Negatives: In many instances in the book, there is not definitive proof of wrongdoing by Rove, just strong cause for suspicion. Unfortunately, the authors don't just come out and say so, and thus come across as "fudging" to make a more damning case. That, and the failure to include perspectives from Rove (who, it seems, mostly refused to comment) or his defenders, makes the book seem more like a "hatchet job" than if they had included these arguments and demolished them.
Also, the book has a strangely patched-together feel, with chapters duplicating each other and no real narrative arc. It's unclear whether this reflects two authors, a book adapted from several articles, or just mistrust that the reader will remember things from one chapter to the next.
The book makes clear that Rove is an evil genius. Unfortunately, the authors are not entirely worthy of their subject. A Rove bio by a great biographer (say, Robert Caro, chronicler of Robert Moses and LBJ) would be a must-read.