I read Mr.Posner's book about JFK and found it quite illuminating. I was a firm believer in a conspiracy before I read the book, but now I belive Oswald acted alone. He presents compelling reasoning and evidence why this is so.
I never read his JFK book, but he co-wrote a very good book on Josef Mengele. Unlike many lefties, he seems able to keep politics out of his non-political work.