A brief but solid biography of an important figure. There are some limitations imposed by the length — for one thing, I suspect previous familiarity with the French Revolutionary period is helpful, as the book rather glosses over some events that would be helpful for context but didn't have much to do with Marat — but as it's one of the only accessible Marat biographies in English that's kind of a limitation you've got to accept. It's got quite a bit of Marat's own voice for the length, which I appreciated, and even if I would have liked some more detail on a few incidents, it does a quite good job of narrating the last four years of his life. The style also makes for a quick, easy read.
Connor definitely has an opinion on his subject he makes no real effort to hide, and I know that will bother some people but honestly it didn't me. No one is without a perspective and this one is owned. His arguments about how Marat's posthumous reputation have influenced the scholarship are well done and definitely set you up for what to expect from the rest of the book. The only time Connor's upfrontness about his perspective bothered me was the counterfactual section right at the end, but to be fair I've rarely found that kind of speculation useful to me.
All in all it's a good choice if you're looking for more information about Marat specifically and to a lesser extent French Revolutionary journalism.