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William H. Gass may identify with “younger writers.” But at 88, he’s firm that he’s written his last novel.

“No more novels. I can’t live forever,” he says, laughing. “Old as I am now, I only plan to write novellas at best, or short stories.”

Last novel isn’t the same as last book, and the St. Louisan has several in the works.

But even if he’s shut the lid on “Middle C,” the novel doesn’t even go on sale until Tuesday. And interviewers have been prying, eager to talk to Gass about his rare production.

He talked last week from his home in the Parkview neighborhood, where he lives with his wife, Mary. Gass, who says poor heart health will prevent future such excursions, has recently returned from a trip to Austria and Germany, where his magnum opus is now available in translation as “Der Tunnel.”

Eighteen years have passed since publication of “The Tunnel.” That book, which had a 26-year gestation, was only Gass’ second novel, even though he’s been publishing books for almost half a century.

When his first novel, “Omensetter’s Luck,” came out in 1966, a New York Times reviewer said of the story set in 19th-century Ohio: “And yet, while the costumes of the book may be historical, its impact is compellingly modern.”

Ever after, Gass has been characterized as avant-garde, innovative or eccentric (thus his identification with younger writers). He’s been credited with the first use of the word “metafiction,” which refers to fiction that self-consciously reveals its status as fiction.

“Middle C,” which is not overrun with meta-playfulness, does include some obvious references. In a list of news items about human violence, for example, a note: “*The reader is invited to substitute or add a similarly focused report whenever this point in the text is reached.”

Yet Gass has never liked being called “postmodern.”

“Writers don’t really like being labeled,” he says. When writers are lumped together, they often aren’t really very similar: “They usually just dislike the same things” (plot, say, or character).

Isn’t it natural to want to categorize writers like species in a biology book?

“We ain’t science,” he says.

What he is is a retired philosophy professor. Gass wrote his doctoral dissertation at Cornell University on “A Philosophical Investigation of Metaphor.” He went from Cornell back to Ohio, where he’d grown up in the town of Warren.

Gass taught for a few years at the College of Wooster, where he remembers the case of a professor who had lied about his credentials (and was a bigamist, to boot). Echoes of that professor’s academic deception are found in “Middle C,” which is also set in a small Ohio town.

After Wooster, Gass taught at Purdue University in Indiana. But he is most associated with Washington University (1969-99), where colleagues included writers Stanley Elkin, Howard Nemerov and Mona Van Duyn.

Although reviewers invariably point out that Gass’ books have “backwater” or “bleak” Midwestern settings, Gass said years ago that he didn’t regret not living in New York. “I am not into that hobnobbing stuff. It’s not what I care about. ... St. Louis is a great place to live,” he said in 2007, when he received the St. Louis Literary Award.

Writing is harder at 88, he admits (joking, perhaps, that sometimes he can’t remember how to spell). He’s working on a book about Baroque prose writers, Baroque writing being something he’s accused of himself, he says. And Gass plans to finish another book of essays called “That Other Art,” and a collection of novellas.

A pair of novellas actually led to “Middle C,” Gass says.

He had two with titles that began with “C” and decided he needed a third for a book. As a kind of joke, he started calling it “Middle C.” Once he had the title, the story grew.

“That’s typical of me. I don’t know where I’m going early on.”

“Middle C” refers, in part, to music, whose structures Gass has used for fiction, evoking mood and emotion and mixing elements the way a musician might. The author plays no instruments but appreciates music, of course. Also, “my prose has been referred to as musical.”

The novel’s main character becomes a music professor even though his immigrant family has little money to buy records or pay for lessons. Joseph also becomes obsessed with evil and gathers evidence for his Inhumanity Museum, which is what he calls his attic of news clippings.

Throughout the novel, Joseph mulls over and rewrites a sentence: “The fear that the human race might not survive has been replaced by the fear that it will endure.”

Gass never appreciates reviewers who try to connect him to his characters and simply notes that Joseph’s apocalyptic musings aren’t “very good advice.”

Essays full of Gass’ own big ideas have often been more pressing work than his fiction and are more desired by editors, he says. The writer has received three National Book Critics Circle Awards for his essay collections. Last year, in a review of “Life Sentences” for this newspaper, John Freeman wrote that Gass has been “America’s best living essayist for an obscenely long time.”

In fact, Gass says, magazine editors will sometimes reject short stories he submits. What reason do they give?

“‘I don’t understand this!’” says the man who spends years writing and rewriting sentences to perfect them.

Now that “Middle C” is published, Gass wants to focus on his current work, such as the Baroque writers’ questions about what angels look like.

“I don’t have a whole lot of patience with writers repeating what everybody else does.”