As the 50th anniversary of his assassination nears, John F. Kennedy remains all but impossible to pin down. One reason is that his martyrdom — for a generation of Americans still the most traumatic public event of their lives, 9/11 notwithstanding — has obscured much about the man and his accomplishments.

Was Kennedy a great president, as many continue to think? Or was he a reckless and charming lightweight or, worse still, the first of our celebrities-in-chief? To what extent do his numerous personal failings, barely reported during his lifetime but amply documented since, overshadow or undermine his policy achievements? And what of those achievements — in civil rights and poverty, to name two issues his administration embraced. Weren’t the breakthroughs actually the doing of his successor, Lyndon B. Johnson?

Even the basic facts of Kennedy’s death are still subject to heated argument. The historical consensus seems to have settled on Lee Harvey Oswald as the lone assassin, but conspiracy speculation abounds — involving Johnson, the C.I.A., the mob, Fidel Castro or a baroque combination of all of them. Many of the theories have been circulating for decades and have now found new life on the Internet, in Web sites febrile with unfiltered and at times unhinged musings.

Of course the Kennedy fixation is hardly limited to the digital world. An estimated 40,000 books about him have been published since his death, and this anniversary year has loosed another vast outpouring. Yet to explore the enormous literature is to be struck not by what’s there but by what’s missing. Readers can choose from many books but surprisingly few good ones, and not one really outstanding one....