That international success set expectations high, but deWitt, who seems as unflappable as his deadpan assassins, has shown no signs of feeling boxed in. His next novel, “Undermajordomo Minor,” was a gothic adventure, and then, in another course change, came a brittle comedy of manners called “French Exit.”

Now, deWitt has published an exceedingly gentle novel about the hushed life of a retired librarian in Portland, Ore. Readers waiting for another book as irrepressible and strange as “The Sisters Brothers” will have to keep waiting. Which is not to say that “The Librarianist” is without charm, only that it presumes a reservoir of goodwill and patience.

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Bob Comet, deWitt’s sepia-toned hero, is 71 years old, healthy, tidy and “not unhappy.” Since retiring from the public library where he spent his entire professional career, he’s enjoyed a life of almost uninterrupted solitude in the house his mother left to him decades earlier. “He had no friends, per se,” deWitt writes. “He communicated with the world partly by walking through it, but mainly by reading about it. Bob had read novels exclusively and dedicatedly from childhood and through to the present.”

That introduction of a life infused with literature signals a kind of Walter Mitty fantasy or perhaps a satire of fiction’s erroneous influence, like Jane Austen’s “Northanger Abbey.” But, alas, despite deWitt’s claim, the Bob we discover in these pages does not appear to have read any particular novels, just many of them, the way Sarah Palin reads lots of newspapers. In fact, even though Bob preaches about the power of literature to show us “the human landscape in all its odd detail,” and to connect us in fraternity with others, we never sense any novels lingering in his mind, shaping his expectations, quelling his sorrows or doing any of the wonderful things that novels do for actual people who read them “exclusively and dedicatedly.” So far as I can tell, Bob could have been equally devoted to cheese or model airplanes with similar results.

And so, from that somewhat artificial premise, “The Librarianist” sets forth at its measured pace.

In the opening pages, Bob discovers a woman about his age wearing a pink sweatsuit in a 7-Eleven. She’s staring off, almost comatose. When he determines that she’s a resident at a nearby retirement center, he gently nudges her back to the facility despite her groans and repeated stoppages. When they finally arrive, the Gambell-Reed Senior Center looks “very much like the clichéd image of a haunted house,” which is in keeping with the novel’s slightly surreal affectation.

Bob’s brief encounter with the errant patient is enough to inspire him to volunteer, which surprises the center’s lax manager. But this provides deWitt with an opportunity to do what he does best: construct funny dialogue involving quirky folks. He’s a master at the way people talk at each other in a series of confrontational repetitions, stepping forward only after skeptically testing each new stone of the conversation.

Surveying the gray snacks on offer in the Great Room, for instance, one of the residents asks the nurse, “How many can I have?”

“How many do you want?”

“How many can I have?”

Such encounters are amusing so far as they go, but the energy picks up considerably — and just in time — in the novel’s second section, which jumps back half a century. DeWitt’s expertise at creating eccentric characters is on full display after Bob gets a job at the library working for a woman who hates children and insists on complete silence. When a cape-wearing fanatic is banned from the building, the man’s daughter (also wearing a cape) begins checking out books for him. Bob notices her — how could he not? — and eventually they fall in love.

The quick progress and fraught terms of their relationship, which can be both funny and poignant, are the heart of the novel. And there’s some outlandish drama here, too — vicious arguments! wild sex scenes! a stabbing! — but all that excitement remains peripheral to Bob and, essentially, peripheral to the plot, too.

“Was Bob exotic in his plainness?” deWitt asks. “Was he merely a straight man?” Bob is sensitive enough to care for the woman he loves but not aggressive enough to fight for her. His passivity is a curious predicament, but such muted ardor poses challenges for a novel. With the protagonist draped in beige, “The Librarianist” must depend entirely on its psychological precision and its stylistic wit, qualities that are measured out carefully here.

A third section of “The Librarianist,” even further in the past, when Bob is 11, promises some fundamental revelation about the formation of his character. But instead we find an extended episode about the boy’s brief encounter with a pair of kooky actresses at a washed-out resort. It’s amusing, yes, but in a way that demands little exertion. Although there’s a whiff of Huck Finn’s adventure with the King and the Duke, without any satiric edge or underlying social implications, it’s just another droll anecdote.

In the end, Bob looks back with satisfaction and gratitude — more of each than I felt. “There had been whole eras of Bob’s working life where he knew a lamentation at the smallness of his existence, but now he understood how lucky he had been.”

A novel about quiet decency in an age short on quietude and decency is nothing to complain about, of course. But simple, decent lives are what most of us lead, so we know that tone well. Given the placid surface of our own existence, an author’s faux plainness grates like a Hollywood actor putting on an accent from our hometown. The late, great Anita Brookner pulled off the harrowing drama of ordinary life in one perfect novel after another. And Stewart O’Nan caught the balanced comedy and peril of old age just right in “Emily, Alone.” But “The Librarianist” never gives us an urgent reason to check it out.

Ron Charles reviews books and writes the Book Club newsletter for The Washington Post.