Precisely how much of the joy one takes in reading J.D. Salinger’s work comes from the knowledge that he has chosen to withdraw from the public eye for the past five decades? The books would be no less beautiful — not a word would change — but if, for instance, Salinger had spent the past few years drawing a paycheck as a staff writer for Vanity Fair, mightn’t that dim the appeal of Franny and Zooey just a bit? Or what if he’d written four sequels to The Catcher in the Rye? What if he’d been, however briefly, the host of a late-night talk show? A guest on The Muppet Show? The center square on Hollywood Squares?