I have now read 3 of the 4 books by Bill O'Reilly in his assassinations series. The latest is Killing Patton, which works on the theory that Patton's death by car accident was, in fact, no accident.
All three books read like fiction. They are fast paced and character driven. They are not complete biographies, they cover the last few years of their lives, leading up to their deaths. The slowest one for me was Patton. The war scenes dragged on and Patton was a bit... unlikeable.
My favorite was Killing Kennedy. I think the authors' memories and emotions of that time were reflected in that book. However, you will also find their political leanings coloring the description of Kennedy and his administration. This seems to be the biggest criticism of these books from other reviewers. Additionally, the authors suggest a conspiracy and cover up of his assassination.
These are easy to read and move at a fast pace. I recommend these to fans of popular history, though some on the left might not be pleased. I have not read Killing Jesus, the other book in this series.