Numberless biographies, of course, have been written about people who also told their own stories; all that’s needed is perspective. And sections of this book do provide striking glimpses into a remarkable life. Weschler resurrects the interviews he did in the early ’80s with Sacks’s friends and colleagues, and with Sacks himself, who illuminates his insistence not merely on the humanity of patients who suffered everything from extreme Tourette’s to severe amnesia, but also on something spiritual within them. “William James,” Sacks tells Weschler about the philosopher known as the father of American psychology, “referred incessantly to ‘the soul’ in his conversation but banished the word from his physiology. … I want to give souls their innings.” A nurse — and nun — at one of the old-age homes where Sacks did rounds puts it more prosaically: “Everyone who reads his notes sees the patients differently, newly. … Most consultants’ notes are cut-and-dried, aimed at the problem with no sense of the person. … With him, the whole person becomes visible.”

Compellingly, Weschler intertwines Sacks’s searching empathy with his sheer strangeness. A colleague on the ward where Sacks met the “Awakenings” patients calls him “deeply eccentric” and describes him as “huge, a full beard, black leather jacket covering T-shirts riddled with holes, huge shoes, his trousers looking like they were going to slide off his body.” A friend from Sacks’s days as a medical resident remembers him as a “big, free-ranging animal” who one day “drank some blood … chasing it with milk. There was something about his need to cross taboos. Back in those days, in the early ’60s, he was heavily into drugs, downing whole handfuls of them, especially speed and LSD.” And, always interlaced, there is Sacks’s own irresistible voice, a concoction of humor and half-concealed torment. “I loved Eryops: clumsy amphibians out of water who took on grace and ease once back in it,” he says to Weschler, as they walk through a natural history museum Sacks visited often in his youth. “I used to have erotic fantasies of all sorts here, and by no means all human (hippos in the mud!).”

By the time Weschler writes of Sacks’s death, I found myself tearing up at the loss of this inspired creature. Yet I read “And How Are You, Dr. Sacks?” with an unshakable sense of missed opportunity. The feeling that the great majority of Weschler’s material has been rendered before — and with more artistic grace — by Sacks himself, in his autobiographies and accounts of treating his patients, could have been mitigated had Weschler chosen to examine topics that Sacks barely touches. As a writer, Weschler should have been intrigued by exactly how Sacks’s books were composed. He would turn in mammoth drafts and wind up publishing spare, elegant narratives; there seems to have been an editorial process along the lines of Gordon Lish’s sculpting of Raymond Carver’s stories. Weschler doesn’t linger on literary process. Nor does he focus on what must have been a crucial force in Sacks’s concern for the neurologically afflicted: an older brother’s schizophrenia. Sacks writes only briefly about his need “to get away from my tragic, hopeless, mismanaged brother”; Weschler might have made this sibling — and Sacks’s flight from him — central to his own explorations.

In his prologue, Weschler sets a high standard for himself when he writes that in his relationship with Sacks he was “a sort of Boswell to his Johnson.” There’s little to be gained by invoking a biography that some rank as the best ever written in English, particularly when this book lacks what Boswell had in his every spirited sentence: a distinctive voice. Weschler calls his book “a biographical memoir,” suggesting a special hybrid, an exciting convergence, an interplay of revelations about Sacks and the author. To fulfill this promise, Weschler would have needed to gaze inward, possibly to expose the kind of raw longing that runs throughout much of Sacks’s writing, nowhere more memorably than in “Awakenings,” when Sacks gives his frozen patients the drug L-dopa and they erupt into hunger, groping nurses and grunting over their food and desperate for love. Instead, Weschler keeps himself forever in check, quoting at length, deferentially, doggedly, from interview after interview, and relegating himself to an almost forgettable role.