Happy ninetieth, Mr. Salinger. So what have you been up to?

Mr. Salinger's disappearing act has succeeded so well, in fact, that it may be hard for readers who aren't middle-aged to appreciate what a sensation he once caused. With its very first sentence, his novel "The Catcher in the Rye," which came out in 1951, introduced a brand-new voice in American writing, and it quickly became a cult book, a rite of passage for the brainy and disaffected. "Nine Stories," published two years later, made Mr. Salinger a darling of the critics as well, for the way it dismantled the traditional architecture of the short story and replaced it with one in which a story could turn on a tiny shift of mood or tone.

In the 1960s, though, when he was at the peak of his fame, Mr. Salinger went silent. "Franny and Zooey," a collection of two long stories about the fictional Glass family, came out in 1961; two more long stories about the Glasses, "Raise High the Roof Beam, Carpenters" and "Seymour: An Introduction," appeared together in book form in 1963. The last work of Mr. Salinger's to appear in print was "Hapworth 16, 1924," a short story that took up most of the June 19, 1965, issue of The New Yorker. In the '70s he stopped giving interviews, and in the late '80s he went all the way to the Supreme Court to block the British critic Ian Hamilton from quoting his letters in a biography.

So what has Mr. Salinger been doing for the last 40 years? The question obsesses Salingerologists, of whom there are still a great many, and there are all kinds of theories. He hasn't written a word. Or he writes all the time and, like Gogol at the end of his life, burns the manuscripts. Or he has volumes and volumes just waiting to be published posthumously.