Sometime early in ninth grade, a substitute teacher saw me reading a John Grisham novel and laughed. I told him I liked the book and he said something like, “Yeah, but it’s not like they’re going to be teaching Grisham in a hundred years.”

I love John Grisham novels. I’ve read everything he’s ever written. Sure, they’re formulaic, but, as I’ve written in the News before, I really like that formula. For what it’s worth, there are actually two formulae: The first is the generic high-stakes legal thriller, and the second is the small-town legal thriller. Virtually all of these latter novels take place in fictional Ford County, Mississippi, a Faulknerian universe unto itself, with a cast of recurring minor and major characters and an endless supply of provincial intrigue. (There are also a few outlying books, such as one devoted to baseball, two devoted to football, a Christmas novel, a memoir, five children’s books — the “Theodore Boone: Kid Lawyer” series — and a nonfiction book. Like I said, I love me some Grisham.)

In the last few years, however, Grisham has departed from his previous formulae to create a new, hybrid model: the activist legal-thriller. His novels are no longer about “just,” say, a fresh-faced Harvard grad working for a mafia-controlled firm, or a fresh-faced Tulane student trying to solve a murder, or a fresh-faced Ole Miss grad stumbling into a twenty-million-dollar case, or a fresh-faced Georgetown grad stumbling into a hundred-million-dollar case. Now, they have a political message.

At first, this message was subtle. “The Appeal” (2008) was about the immense problem of electing appellate judges. “The Associate” (2009) was about the soul-sucking perils of Big Law. “The Confession” (2011) was about the iniquities of the death penalty. “Sycamore Row” (2013) was about the prevalence of racism in the legal system.

But Grisham’s latest novel, “Gray Mountain,” published in October, goes way further than any of these. It also breaks new ground for Grisham’s literary activism: environmentalism.

“Gray Mountain” tells the story of Samantha Kofer, a fresh-faced Columbia graduate who is laid off by her Wall Street mega-firm in the midst of the 2008 financial crisis. Kofer is not completely cut loose, however; she can keep her benefits and have a shot at re-employment if she works pro bono for a year. So Samantha sets off for the Mountain Legal Aid Clinic, a tiny public interest firm in rural Brady, Virginia.

“Gray Mountain” has all the usual Grisham themes of an unlikely-yet-oh-so-predictable love interest, a diamond-in-the-rough case and the charms of small-town America. Samantha gradually grows to appreciate her time in Brady, falling for its unadorned beauty. She helps poor people fight against ruthless lawyers, intransigent bureaucrats and abusive spouses. She meets Donovan Gray, a man straight out of a soap opera: the ruggedly handsome, smart, tough, earthy, orphaned, fierce lawyer with a heart of gold, lungs of coal and a tragic past. When Samantha meets him, Donovan is in the midst of a series of suits against Big Coal. In one, a strip-mining company accidentally dislodged a boulder that crushed two small children, killing them in their beds. In another, the chemicals used by a mining company have created a cancer cluster in a small town. Meanwhile, Samantha finds a cause of her own: an apparent epidemic of “black lung” among poor, unlettered miners.

But “Gray Mountain” also departs from the Grisham model in a few important ways. First, a main character unexpectedly dies. Second, there is no climactic case — in fact, no major case is resolved; many are left annoyingly open-ended, even more so than usual. And, most importantly, the plot ends up playing second fiddle to Grisham’s furious and noble vendetta against Big Coal.

Much of “Gray Mountain” amounts to an unrestrained jeremiad against the mining industry. Grisham decries the evil companies that mislead Appalachians and then abuse their land. He points out the countless instances of murder, the jurists and politicians in the pockets of industry and the environmental desecration that is fundamentally changing Appalachia. In doing so, Grisham runs the risk of losing the story, while retaining nothing but his politics. Yet he doesn’t quite go so far. The novel has an unusual plot and no clear ending, but its characters are engaging, its scenery beautiful and its story cohesive enough to hold interest.

“Gray Mountain” is a good read, as always. But it also brings light to a largely unknown, massive problem. It is a public service. It is the reason we just may be reading Grisham in a hundred years.

“I was drawn to the topic of mountaintop removal because it is an ongoing environmental disaster, destroying much of the culture of Appalachia,” Grisham said in a recent interview. “And I’m not finished with it.”

And I’m certainly not finished with Grisham and his exciting new activism (whatever my substitute teachers might say).