There is a moment, pages from the close of Ingeborg Day’s 1980 memoir, “Ghost Waltz,” that will cause anyone who writes for a living to cringe. Day, then thirty-eight, is attending a neighbor’s bar mitzvah—marvelling at the Saturday service, at the boy’s speech, at the boisterous sense of community at the family luncheon. Then a large man squeezes his bulk next to Day and booms, “Ingeborg, my wife wants us to trade seats, she loves writers and wants to talk to yoooo-hoooo!”

Day thinks of faking an upset stomach, a leg cramp, a missed appointment, but there is no escaping this encounter. “My husband is right,” says the wife. “I do love writers. Now tell me all about it, what kind of book are you writing?”

Panicked, Day gives her answer: “It’s, I’m, uh … it’s an erotic epic poem.” And then, much to her immense relief, everyone around her bursts out laughing.

In the context of “Ghost Waltz,” which recounts Day’s agonized grappling with her Austrian father’s Nazi past and his refusal to discuss it, this anecdote about the “erotic epic poem” plays like uncomfortable comedy, too strange to be true. Except it turns out Day was telling the truth: she was about to publish a manuscript that became “Nine and a Half Weeks,” a notoriously extreme account of a sadomasochistic relationship which was released under the pseudonym Elizabeth McNeill in 1978.

At a moment when erotica is mainstream, and when the issue of who controls a woman’s body has never been more relevant, “Nine and a Half Weeks,” and its frank, poetic treatment of illicit sex, has lost none of its disturbing power. But what’s stranger is that Day chose to reveal even more disquieting aspects about herself under her real name than she ever did with a nom de plume. Examining “Ghost Waltz” and “Nine and a Half Weeks” in tandem, we see a woman whose identity is divided in two.

As an inveterate lover of mystery, cracking the code of a writer’s true identity has the same effect, for me, as tasting forbidden fruit. In looking further into Day’s story, I hoped to learn more about who she really was. But what I discovered raised more questions than answers—which is likely what Day would have wanted.

“Nine and a Half Weeks” is a potent antidote to what passes for erotica today. Instead of over-the-top fictional fantasy, McNeill’s book, presented as memoir, is charged as much by explicitness as it is by absence. The reader is only privy to her perspective, and even then, it’s occluded by the use of a pseudonym.

“The first time we were in bed together he held my hands pinned down above my head. I liked it. I liked him.” McNeill, from the first, avoids exposition. The reader is made complicit in the tension between her and her unnamed lover—a tension that at first seems exciting but quickly becomes fraught and frightening. A paragraph later, she describes the second time they are together, and his simple, seductive question: “Would you let me blindfold you?” She agrees, and he does, with the scarf she dropped on the floor, The third time, she has to beg him to bring her to orgasm after he stops her, multiple times, before the brink. By their fourth encounter, “when I was aroused enough to be fairly oblivious, he used the same scarf to tie my wrists together. That morning, he had sent thirteen roses to my office.”

The book is just under a hundred and twenty pages, but its stripped-down presentation of sadomasochism is so vivid, the images so searing, that to draw out its tensions any longer would be more than most readers could bear. Early in their relationship, McNeill’s lover brings a shaving mirror into the bedroom, slaps her face, holds her by the hair, and forces her to look at the symmetrical mark on her cheek. “I stare at myself, mesmerized. I do not recognize this face; it is blank, a canvas there to display four smudges, red like war paint. He traces them gently.” At night, McNeill spends hours with her arm chained to a couch; or goes with her lover to buy a whip he tests in public on her bare legs; or is instructed to go to a five-star hotel room to put on man’s clothes, playact the gender switch in the lobby, and then return to the room where her lover “takes me like a man.”

McNeill, described in press notes at the time as a New York-based executive at a large corporation, is apparently able to dissociate her competent working self from the sexual slave she has become to her increasingly demanding, dominant, borderline abusive lover. She recognizes, of course, that the reader may be horrified by what she’s describing, but she leavens the melodrama of the story by writing with near-clinical detachment:> Throughout the entire period, the daytime rules of my life continued as before: I was independent, I supported myself (to the extent of my lunches, at any rate, and of keeping up an empty apartment, gas and phone bills at a minimum), came to my own decisions, made my choices. The nighttime rules decreed that I was helpless, dependent, totally taken care of. No decisions were expected of me, I had no responsibilities. I had no choice.

I loved it. I loved it, I loved it, I loved it, I loved it.

For the discrete time period of nine and a half weeks, she keeps to the day/night separation, her lover’s demands grow more alarming and her orgasms become predictable, “like a well-made windup toy.” Only much later, when McNeill has a mental breakdown that marks an abrupt end to the relationship, can she understand her state of mind: “That it was me who lived through this period seems, in retrospect, unthinkable. I dare not look back on those weeks as on an isolated phenomenon, now in the past: a segment of my life as unreal as a dream, lacking all implication.”

“Nine and a Half Weeks” is an intense and compressed reading experience. (The 1986 movie version, with Kim Basinger and Mickey Rourke, which keeps the bare bones of plot but not much else, is far inferior artistically.) It largely achieves its goal of self-containment, but the author’s hidden identity remains tantalizing. We want—we need—to know more about the woman who called herself Elizabeth McNeill. Why did she fall into this relationship? How could she? Was there more to her story? “Ghost Waltz” offers answers, but not necessarily the ones we thought we wanted.

McNeill’s true identity as Ingeborg Day was first revealed by Steven M. L. Aronson in his 1983 book “Hype,” which is about the ways in which public figures transform themselves, physically and existentially, to satisfy the marketing machine. It was confirmed separately to me by several sources, including Day’s longtime literary agent, Wendy Weil, just two weeks before Weil died in mid-September.

Knowing that McNeill is really Day is like opening a door but not being able to shut it properly. The hinges don’t quite fit together as they’re supposed to. The revelation invites us to speculate as to whether Day’s breakdown spurred her to dig into her father’s history, and whether that unconscious knowledge primed her for the sadomasochism described in “Nine and a Half Weeks.”