“My name is Kinsey Millhone. I’m a private investigator, licensed by the state of California. I’m thirty-two years old, twice divorced, no kids. The day before yesterday I killed someone and the fact weighs heavily on my mind.”

That is how Sue Grafton introduced her doughty P.I. in A Is for Alibi, the book that launched her alphabet series in 1982. Twenty-three bestselling books later, Kinsey has solved scores of crimes, many of them violent. She has come within inches of death multiple times. She has not remarried but has had several complicated relationships. In the real world, Grafton, who was 42 when Alibi was published, is now 75 years old. In X, on the other hand, Kinsey is only 38.

It’s March 1989 in fictional Santa Teresa, California (a town whose coordinates, if available, would probably point you to Santa Barbara), a long-ago time when some people still used index cards for research, typewriters for writing, the postal service for delivering messages, and landlines for speaking with friends and colleagues. It’s not an entirely unfamiliar world: California is suffering a serious drought, and Kinsey’s 89-year-old landlord, Henry Pitts, is obsessed with finding ways to trim his spiraling water bill.

Kinsey, annoyed by Henry’s sunburned yard and plumbing innovations, is even more exasperated by his capitulation to their new neighbors, an elderly couple who have suckered him into buying their groceries and running their errands. Rebuffing their attempts to get her to help too, she focuses instead on a widowed friend who urgently needs to find missing financial documents before the I.R.S. audits her late husband. In addition, she has been hired by a wealthy divorcée to find a long-lost son, a job that looks simple at first but that quickly turns complex and time-consuming.

Meanwhile, in a pricier part of town, Teddy Xanakis seems to be planning a heist against her ex-husband, Ari.

It’s all in a month’s work for Kinsey, who recently came into a fairly large sum of money and could take some significant time off if she wanted to. But, she tells us, “anyone who knows me will testify to how ill-suited I am to a life of leisure. When it comes to work, it isn’t so much what we do or how much we’re paid; it’s the satisfaction we take in doing it.” Kinsey takes enormous satisfaction in chastising cheaters.

“My quest for law and order began in the first grade when I ventured into the cloakroom and surprised a classmate snitching a chocolate bar from my Howdy Doody lunch box,” she explains. “The teacher appeared at that very moment and caught the child with my candy in hand. I anticipated due process, but the sniveling little shit burst into tears, claiming I’d stolen it from her. She received no punishment at all while I was reprimanded.… From that singular event, my notion of fair play was set, and, in sum, it is this: the righteous are struck down while the sticky-fingered escape. I’ve labored all my life to see that justice plays out the other way around.”

Kinsey’s dogged quest for justice has a lot to do with why the alphabet series numbers millions of copies in more than two dozen languages, why X jumped to first place on the New York Times hardcover fiction bestseller list just weeks after its August 25 publication, and why I put a library hold on each forthcoming book just as soon as I learn its title. Researchers say we may be born with a sense of fairness. By the time we start preschool, “no fair” is one of our favorite expressions. As adults, we fume if we think politicians are giving an unfair advantage to the rich—or to the poor. We think we want justice, and justice is what crime writers (Patricia Highsmith excepted) offer us.

Unalloyed justice, however, is not necessarily what anyone craves after a hard day’s work. Fortunately, Kinsey’s righteous pursuits are liberally spiced with wit and humor. It’s hard not to love her. She’s funny. She’s sassy. She eats junk food and cuts her hair with nail scissors. She lives in a repurposed garage. Though she thinks of herself as a loner, she’s fiercely devoted to the genial Henry and his odd assortment of siblings.

But when Kinsey smells trouble, she springs into action; and with Kinsey on their tail, the wicked cease to prosper. “I’m getting more male readers,” Grafton told Allen Pierleoni at the Sacramento Bee, “because I say to guys, ‘This isn’t about mascara and panty hose, it’s about kicking serious ass.’ They go along with that.” Yet even when pursuing a serial killer, Kinsey seems more human than Jack Reacher, for example, or Philip Marlowe. She is not a killing machine, and her signature drink is cheap Chardonnay.

Grafton writes so well that it’s easy to overlook her spare and elegant style. Her prose is simple and unobtrusive, moving the story along without calling attention to itself. Her words are ordinary but precise. Her sentences have rhythm. She uses descriptive detail effectively to create a sense of place. Even her minor characters have individual quirks.

Her books are generally well researched (who knew that gray water had to pass through healthy topsoil to avoid bacterial contamination?), though if Kinsey were a practicing Catholic, she might have found an additional reason to distrust Father Xavier’s recollections. The priest tells her that, back in 1961, a character named Pete briefly considered converting to Catholicism, and he says he encouraged Pete to join “the group class we offer on the Rite of Christian Initiation of Adults.” Apparently the priest forgot—or Grafton did not realize—that in 1961 there was no RCIA. It was a product of Vatican II, which had not yet begun, and was rarely used in Catholic parishes until the late 1980s. Fortunately his anachronism is only a venial lapse, peripheral to the story and easily forgiven in this arresting tale that is among Grafton’s most satisfying.

A mystery is literary comfort food, because we can be pretty sure that before the book ends, most of its puzzles will be solved. With this one, though, one mystery remains: why did Grafton drop her usual formula and call it simply X? “I’ve checked the penal codes in most states and xylophone isn’t a crime, so I’m stuck,” Grafton quipped two years ago. To be sure, there are a lot of x’s in the story: the Xanakis couple; their portage company, XLNT; a painting of a xebec; an X on a banker’s box; the Bank of X. Phillips; Father Xavier. There are quite a few exes as well. The story begins with Teddy plotting revenge against her ex, and three of Kinsey’s ex-boyfriends make cameo appearances. None of the x’s (or exes) apply to the whole book, however. Eventually, Grafton told Jocelyn McClurg at USA Today, “it occurred to me that since I was the one who invented this 'rule' about '… is for … ' I was surely entitled to break it.” “I think it's best,” she said to novelist/blogger Mark Rubinstein, “if X represents the unknown.”

So far there’s no word on the street about what, if anything, Y represents, though Grafton long ago revealed that Z Is for Zero. By 2019, when the alphabet series is complete, Kinsey will be almost 40 and Grafton will be 79. Will she continue to write? Like Kinsey, whom Grafton has said “is herself, only younger, smarter, and thinner,” Grafton may be ill-suited to a life of leisure. Her legions of fans, myself among them, hope that’s true.

LaVonne Neff, who lives a stone’s throw from America’s second-most violent city, blogs at Lively Dust.