Having finished Fatal Purity, I wouldn't recommend it in particular. I've read a lot of biographies of Robespierre and this one not only brings nothing particularly new to the table, it doesn't keep in mind what others have had to say before it or make much use of the newest studies. Moreover, Scurr makes some questionable judgments on her subject that don't seem paricularly well-supported. Still, with the possible exception of G. Rudé, you're not likely to find much better in English, unfortunately.