The valuable thing about Gerald Posner is that one can usually discern the truth of any subject simply by the diametric opposite of what Posner believes. This is his true contribution to the world.
In the case of his book 'Cased Closed' on the Kennedy assassination, that would be 'Case Open' by Harold Weisberg,
http://www.amazon.com/Case-Open-Unanswe ... 855&sr=1-1
but my personal favorite is 'Final Judgment' by Mike Piper.
http://www.amazon.com/Final-Judgment-Mi ... 298&sr=1-1