Stephen King loves writing. That much is clear from his sheer output over the past four-plus decades, which is all his own work. Plenty of authors at his level of fame would be happy to hand over the reins to ghostwriters and coast on their royalties. Beyond that, though, Stephen King loves to write about writing. “The Shining,” which was his third novel way back in 1977, introduced the world to Jack Torrance—one of the most infamously frustrated writers in literature and cinema — and the trend has continued ever since, up to and including his latest novel, “Billy Summers.” A love letter to the transformative effects of putting words on paper that masquerades as a boilerplate action-thriller, the novel doesn’t always fire on all cylinders, but the impassioned argument Mr. King makes for the role of writing in healing traumas is heartfelt and affecting.

As the novel opens, the eponymous protagonist, a 44-year-old ex-Marine-sniper-turned-hired-assassin, is offered the highest-paying contract of his career: two million dollars to wait for an accused murderer to be extradited to a small city somewhere along the southern border, where Billy will assassinate him from an office building down the street.

As a precaution, the clever and analytical Billy always takes care to present an uneducated, slow-witted façade he calls his “dumb self” to his employer, believing it safer if the dangerous and powerful men he works for underestimate him. Though he finds himself immediately suspicious of the discrepancy between the hefty sum he is being offered for the job and its relative simplicity, he does eventually accept the contract; not just for the ample money to kill someone whom Billy is assured is a “bad man,” but also because part of the plan requires that Billy — an avid reader — poses as a writer working on his first book, while he waits for his target to arrive.

Billy settles in to wait. In the meantime, he befriends his new neighbors around his temporary home, shares lunch with other workers at the office building, and begins to write. While he chooses to write in that same “dumb self” voice just in case his employers are tracking his laptop, the story he tells is a painfully honest autobiographical recounting of his traumatic childhood. As he writes, the memories become clearer and his recall of details sharpens, and he finds the effect intoxicating, even when it is painful.

Months pass in this way, with Billy writing regularly, spending time with his neighbors — and as his suspicions pile up, secretly concocting his own escape plan. Once his target finally arrives, Billy must prepare to do the job he’s being paid for and then disappear immediately afterward — with a thumb drive containing what he has written so far.

There wouldn’t be much of a novel if everything went off without a hitch, of course, and what unfolds both during and after that climactic moment is fast-paced and cleverly constructed, even if it isn’t entirely original — there’s a whole lot of Luc Besson’s 1994 cerebral action film “The Professional” here, and maybe even more of Antoine Fuqua’s 2014 vigilante movie “The Equalizer.”

Even if the capital “T” thriller portions of the book feel a bit derivative, they are still written with Mr. King’s legendary eye for detail, and his ability to immerse readers in the mindsets of fictional characters serves the story well. The fans he refers to as Constant Readers may also find themselves smiling at some late-book references to a certain haunted hotel in Colorado, but otherwise, supernatural forces are absent from “Billy Summers,” putting it more in the tradition of the three intentionally-pulpy crime novels he has written for the Hard Case Crime imprint over the last 15 years (including “Later,” which was released earlier this year).

There are some missteps: for example, Mr. King occasionally tosses in foreshadowing about COVID-19, even though it has no bearing on the plot and none of the book actually takes place during lockdown, which is puzzling. On the other hand, the book shines the most when it talks about the act of writing. Billy is able to process his myriad traumas through the act of remembering his past and writing about it, and witnessing this deeply-scarred man discover a new way of seeing himself and his place in the world is beautifully resonant. Mr. King’s sheer pleasure in the alchemy of turning mere words into entire universes is on full display here, and it is contagious — not just for Billy, but perhaps for Constant Readers as well.