“All I knew was: This is unfinished business,” Spiegelman tells The Post’s Comic Riffs by phone from France, while traveling to a festival.

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He drew the comic, but the depth of this powerful personal wellspring did not ebb. Several years later, Spiegelman moved back East. “I went to visit him with a tape recorder,” the cartoonist says of his father, Vladek, who lived in the Rego Park neighborhood of Queens, “and figured this would be the start of something.”

“Something” turned out to be “Maus,” the Pulitzer-winning graphic memoir, a landmark project that led the American public, including many literary critics, toward seeing comics as a serious art form. Tomorrow marks the 30th anniversary since the first collected volume, “Maus, a Survivor’s Tale: My Father Bleeds History” (Pantheon Books), was published.

Today, amid the massive boom in graphic novels, it can be easy to forget how much of a game-changer “Maus” was.

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The comic installments ran in serial form in RAW, the indie “graphix” magazine launched in 1980 by Spiegelman and his editor-wife, Francoise Mouly, now art editor at the New Yorker. That’s where rock-star cartoonist Chris Ware (“Building Stories”) first read it. “Probably more than any other single comic, it made me see not only the potential for complex, moving and intelligent storytelling in comics, but also galvanized my own resolve to become a graphic novelist,” he says.

The series inspired another budding young cartoonist in the ’80s, acclaimed “Bone” creator Jeff Smith. “It was a big deal. My comics pals and I were blown away,” says Smith, who first met Spiegelman while a student in an Ohio State University class, in 1986. “Nobody had ever seen anything like it.”

“The most brilliant thing was, this same person who came up with Wacky Packages and Garbage Pail Kids [cards] now decided to depict the Nazis as cats and Jews as mice. Wow. This was in equal parts outrageous, disturbing, subversive, witty — and yet somehow comforting,” says Smith, who is director of the forthcoming Cartoon Crossroads Columbus festival in Ohio. “By using talking animals, Spiegelman allows his readers just enough emotional-safety distance to be able to follow a story that takes place during the Holocaust. Before you know it, you are with Vladek, unmoored and slipping into the cruelest pits of hell.”

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Yet Spiegelman only gradually realized what a long and arduous endeavor it would be — just as publishers didn’t initially realize what a literary accomplishment he was gradually carving out.

“In the beginning, I thought: This will be about a two-year project,” says the cartoonist, who, between each of the book’s chapters, would take on a less creatively taxing assignment. It took him 13 years from conceiving of it as a book to its completion.

After “Maus” comics had appeared in the biannual RAW for about five years, Spiegelman pitched the work to publishers — meeting little enthusiasm. Then, in 1985, Ken Tucker’s essay for the New York Times hailed the work-in-progress as “a remarkable feat of documentary detail and novelistic vividness,” and it quoted a Library of Congress graphic-art curator who said that the cutting-edge “Maus” returned “an excitement that has been lost in comic art.”

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“The result,” Spiegelman says, “was all of a sudden, Pantheon was interested in putting out the first volume.”

It landed in August 1986, inspiring a generation of up-and-comers who held no prejudices about the comics form.

“I first read ‘Maus’ in my late teens,” says Gene Luen Yang, a literary ambassador for the Library of Congress whose 2006 “American Born Chinese” would become the first graphic novel to be named a National Book Award finalist. “Art Spiegelman set the standard for the rest of us . . . He gave us something to chase after.”

Spiegelman created “Maus” as a “frame tale,” depicting his ’70s conversations with his father as a contextual window into the World War II experiences of Vladek and his wife, Anja — both Polish Jews who were persecuted by the Nazis. Anja’s mental-health issues are foreshadowed in “Maus”; she committed suicide several months after son Art left a state mental hospital, after a brief stay, in 1968.

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Spiegelman drew “Maus” in black-and-white hatched panels, intentionally using a simple style that heightens the blunt impact of the content. And the cartoonist deftly employs many subtle tricks and literary devices — from visual foreshadowing to well-timed flashbacks — that gather cumulative force. “Maus” won the Pulitzer Prize in 1992, after the publication of its second volume.

“I had no idea the book would have the impact and resonance that it’s had since,” says Spiegelman, whose shorter works include “In the Shadow of No Towers” and “Breakdowns.” “My mission wasn’t to make a long comic book that needs a book mark.”

(And it’s worth noting: The legendary Will Eisner, author of “A Contract With God,” had popularized the term “graphic novel” some years earlier, and Spiegelman now champions such early “wordless novelists” as Flemish wood-cut artist Frans Masereel and American Lynd Ward.)

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Some, like Smith, wish that Spiegelman would do more long-form comics. Spiegelman says “Maus” was the one time he felt compelled to write one. “Some people want a ‘Maus’ 3, 4 or 5,” says Spiegelman, laughing. “But that’s it.”

And really, what more needs to be said?

“It continues,” Ware says, “to be the greatest graphic novel ever written.”