I rarely read nonfiction, but this New Yorker article discussing the French Revolution and "the rise of the mass-murdering nerd" piqued my interest, so I picked up Fatal Purity at the Clerksville library.
First things first: you don't need to know a lot about the revolution to enjoy this book. All I was working from were some vaguely remembered high school history lectures and a bunch of paintings by David. Scurr does a good job of summarizing the action and the shifting factions so even if you didn't know your Hebertists from your Girondists before, you can keep tabs on what's going on.
There were three things that bugged me about what is otherwise an excellent and informative biography of a relatively neglected historical personage. First, Scurr uses many large block quotes from Robespierre's speeches. These get repetitive, not least because all of them make essentially the same argument (France has secret internal enemies and I know who they are! The state must crush them!).
Second, Robespierre had a fiancée! This was big news to me, and could have been a great window into the man if explored in depth. Unfortunately, there's more in Wikipedia about the so-called "Widow Robespierre" than there is in this book. Maybe he got engaged to someone he didn't care that much about. Maybe there aren't enough source materials to shed light on the relationship. But after telling the reader all about how other people's wives and families followed them to the guillotine, to not even mention her fate was odd.
Finally, and this is not Scurr's fault: Robespierre is kind of a boring guy. He twitched, he made speeches, he schemed, and he condemned people to die. But there's no there there. Most mass murderers have more personality.
Nevertheless, this is a decent book if you're interested in the French Revolution, politics, or why basing the legal system off of books* you read as a teenager is a bad idea.
* The works of Rousseau