In 2017, many American women—the rich, white, and sheltered conspicuous among them—turned outward, enlisting in causes greater than their families or themselves. Jessica Simpson, the pop star turned fashion mogul, was at the time in the throes of an addiction to alcohol and sleeping pills (having quit stimulants a couple of years earlier, after a doctor warned her that they could kill her). Her new memoir, “Open Book,” is a mesmerizing record of this and other problems, including the fact that, as Simpson acknowledges, “There’s always a lot of people at my house.” On the Halloween afternoon that leads her to realize that she needs to address her issues, the crowd includes both of her parents; her house manager, Randy (“my dad’s best friend from when I was a kid”); workers setting up for a party; a “glam squad”; however many guests would require the twenty golf carts Simpson and her husband have rented to go trick-or-treating in their gated community; and Evelyn, her housekeeper of fifteen years, who finds her crying in her bedroom and holds her, stroking her forehead, until she zonks out. The next day, amid more hairdressers, Simpson’s friends, who also work for her, stage an intervention. Part of her cure turns out to be writing. She returns to the diaries that she’s kept since adolescence, organized—“a black one for the end of my first marriage, red for the hope of a love affair, blue for when I wanted to focus on my career and song lyrics”—according to a Dewey Decimal System of the heart. Journaling, the rare creative activity that brooks no collaboration, requires Simpson, as much as anything, to spend some time alone. “I think it’s important, whatever your situation, to turn inward,” she concludes, sober, now, for two years.

“Open Book” has been phenomenally successful, reaching No. 1 on the New York Times best-seller list and compelling people who couldn’t previously have cared less about Simpson to, as one Goodreads commenter wrote, “Google enough about the 2000s to throw off the algorithm.” Simpson, whose clothing line grosses a billion dollars a year, knows how to make product move. The gossip in the book is fluorescently transparent and sweetly viperous, like a pink snakeskin sandal with a PVC upper. We learn that Simpson’s first husband, Nick Lachey, demanded some phenomenal sum of alimony as they were divorcing, which she eventually paid to make him go away, but not before sleeping with him one last time, after which she fell in love with John Mayer, who kept dumping her by e-mail and eventually coaxed her entire entourage into supporting a reconciliation (“Randy believed him”), only to bail after she broke up with Tony Romo, her boyfriend at the time, who unfortunately didn’t want her to take any movie roles that required onscreen kissing. (We won’t delve into her liaison with Johnny Knoxville, except to say that his nickname for her was “lady.”)

Even the less juicy moments of Simpson’s life are often fascinating, as a sort of ethnography of X-ennial Los Angeles fame. As participant-observer, Simpson is both aware of and oblivious to the weirdness. “ ‘This guy just left Jessica Simpson naked in bed to go see Marianne Williamson,’ I said aloud,” she writes, of the morning after she met her current husband, Eric Johnson, a former tight end in the N.F.L., with whom she has three young children. The accumulation of detail creates an intimacy with the reader that, unusually, isn’t based on a disingenuous kind of levelling. Hooking up with their new crushes and then referring to themselves in the third person: stars are only sort of like us, Simpson acknowledges. We don’t identify with her because she once spent “something like a hundred and fifty thousand dollars” on her husband’s birthday weekend in Las Vegas; we identify with her because she can’t keep herself from telling us about it.

We’ve known since the days of “Newlyweds,” her MTV reality series, that Simpson is a talker. She’s a writer, too, entrancing and somehow economical, even in a book that runs to four hundred pages. She has an insane gift for the thumbnail sketch. Who, now, will be able to look at Justin Timberlake, who competed against Simpson in 1993 for a spot on “The All-New Mickey Mouse Club,” and see anything but a boy “running around the pool, completely ‘on’ like he knew the audition finals had already started”? Who needs more than “Stephen and Mary Jo are from Boston and are very New England in their demeanor” to guess how Simpson’s early encounters with her in-laws probably went? Until now, Simpson’s most immortal one-liner involved tuna fish. But her recollection of seeing a car belonging to her then estranged father, a former Baptist preacher, in the parking lot of her daughter’s school—“It was hard to miss, a bright-green custom sports car I recognized from his Instagram”—is just as unforgettable, a sixteen-word novel for the age of self-actualizing stage dads.

Simpson worked on “Open Book” with a ghostwriter, Kevin Carr O’Leary. Whatever their process was, her own flair for language is apparent, from her neologisms (“glittercups” are the gold, lidded tumblers from which she drank vodka and flavored Perrier at all hours of the day) to bits of dialogue that have clearly tickled her and her family for years (“You gonna be doing acrobatics in the church chapel?” a deacon asks her mother, who is trying to start an aerobics class called Jump for Jesus). Her depictions of fashion on the brink of Y2K are vivid enough to render a search engine moot. She recalls Nick Lachey, on the night they met, in “red overalls with the left strap off, and a cream turtleneck.” In a new documentary, “Miss Americana,” Taylor Swift theorizes that celebrities often get mentally stuck at the age they became famous. Simpson just seems to have got stuck in the outfit. The night that Johnson proposed to her, she recalls, she was watching television on the couch in his “big Yale sweatshirt” and what she likes to call her “ruffle-butt panties.” Her book-tour looks have marked a return to glamour, but I would bet money that she still has a pair of Bongo short shorts hanging around somewhere.

If Taylor Swift is the liberal whom people long suspected of being a conservative, Jessica Simpson is the conservative who might be becoming a liberal. By the end of the book, she has come to terms with a lifetime of traumas, from the micro (Chuck Norris taping her eyebrows in an acting class) to the medium (being fat-shamed for wearing a pair of “mom jeans”) and the major (childhood sexual abuse and the horrific bullying that followed it). The transformation seems to involve some turn, however vague, toward a form of feminism. “How many times are women made to feel responsible for the actions of men?” Simpson writes, of a post-divorce encounter with Lachey. “I know now that I wasn’t, but back then, it felt like I needed to fix him.” Her vocabulary, frequently so precise, is vague here. Likewise, the only time in the book that she participates in the ritual of checking her privilege, her intent is unclear. “I was very aware of my privilege in my ability to go home,” she writes, of performing for troops in Tikrit, Iraq. She adds, of another U.S.O. gig, “I chose that song because I believed they should all be able to come home.” Why does she believe that? For once, she doesn’t say, leaving the reader bewildered as to whether she’s just read “The Secret” or is making some kind of antiwar statement, however meek. She is also oddly evasive about race, especially given that she began her career in gospel music. “They seemed to be just as surprised to see me as I was to see them. Like, Who is this little girl?” Simpson recalls, of a recording session with a full gospel choir, organized by her early mentor Dr. DeForest B. Soaries, the pastor of the First Baptist Church of Lincoln Gardens, “one of the largest African-American congregations in New Jersey.” She seems to be holding back from saying what you can more plausibly imagine the gospel singers asking themselves: “Who is this little white girl?”

I still have a few questions: Is it just a coincidence that Simpson’s good friends are named CaCee, Lolo, and Koko? Why do her kids have to be at school at seven o’clock in the morning? But “Open Book” is ultimately a testimony—a gut-spilling account of getting saved all over again and understanding the responsibility that grace implies. Whatever her other evolutions, Simpson clearly remains a child of God, who, she writes, had “given me a light and a calling to use my voice to help others.” She is perceptive about honesty, which, she observes, “will lead you somewhere.” She writes, “When people saw the real me, they wanted me to succeed.” I see her now, succeeding, and I wonder how she wants to use her money, her intelligence, and her time. In 2017, something awoke in Simpson. She can speak honestly now. Is there anything else she wants to say?