The artist who co-created Clark Kent and Superman had his own secret identity — as an illustrator of sadomasochistic pulp books.

“This stuff was highly illegal — you couldn’t mail it across state lines,” said artist and comics historian Craig Yoe.

In his new book, Secret Identity: The Fetish Art of Superman’s Co-Creator Joe Shuster — due to hit store shelves in April — Yoe describes finding a copy of the pulp porn Nights of Horror in a used bookstore.

Seeing the unsigned pictures in the book, his immediate reaction was “Oh my God, Joe Shuster.” The Superman artist’s work was like a fingerprint, he said, adding heads of the comic industry had the same reaction to the artwork in the pulp novel.

Yoe says the story behind the Nights of Horror artwork, which he chronicles in his book, is as lurid as the drawings: Shuster’s bitter departure from D.C. Comics and the Superman he created, his two decades in obscurity, dirty cops, showgirls, Brooklyn thrill killers, bullwhips and a senate investigation that nearly collapsed the comic book industry.

“It’s too crazy for comic books,” Yoe said.

Some of the pictures remain unnerving today, he adds. But Shuster’s illegal artwork — which had 80 detectives searching for its creators — still adhered to certain odd standards of the time.