Otto Penzler's library looks like the one where the butler did it.

*The desk chair stands against the wall facing the room's only door. Two tall windows let in the steady but chilling north light to shadow Penzler and focus on the suspect. The desk itself is large enough to hold the requisite body-in-the-library stabbed with the heirloom paper knife or brained with the hereditary inkwell. A blanched skull sulks on the sofa table, waiting, perhaps, to be avenged.

It's a fit setting for the Mystery Mogul of Manhattan -- publisher of The Mysterious Press and The Armchair Detective magazine, founder of the Crime Collecters Club, coeditor of the "Encylopedia of Mystery and Detection," collector of rare mystery books, proprietor of the Mysterious Bookshop and the suspense fan of all time.

Not satisfied with the quality and quantity of mystery books available, Penzler has become a collector of mystery writers as well. In antics worthy of his turn-of-the-century hero, Raffles, the "Amateur Cracksman," Penzler purloins authors he admires from major publishing houses. His small but burgeoning presses have become home to Donald Westlake, Ed McBain, Patricia Highsmith, Joe Gores, P.D. James, Gregory Mcdonald, Anthony Price and Elmore Leonard. "I would kill to meet Charles McCarry," he said.

Mystery writer Westlake says his friend Penzler's success in publishing and selling books, fields notable for failures, comes "not because he's a hard-headed businessman. He isn't. He's one of God's fools. He's lucky, which in the long run is better than being street smart. It takes him six months to find out that something can't be done -- and by that time, he's done it."

*Publishing has always been full of crusaders for one cause or another, but Penzler may well be the first to take up suspense stories as holy war. He likes mystery writers so much he even started a separate publishing house, Penzler Books, just to issue their nonmystery books. The catalyst was Westlake's satire on the publishing business, "A Likely Story." The novel had been turned down by six publishers, including one who said, "Humorous fiction is my idea of small, if you know what I mean." The rejecter is now bankrupt, but the book, which sold well, has been optioned for the movies.

"In the 20th century, mysteries make up a large percent of what is great literature," Penzler said. "I started publishing mystery writers because as a reader and collector I was upset by the condescending attitude of publishers toward mysteries. Publishing is so often an exercise in pomposity."

With this philosophy, Penzler starved for years, but now his long enthusiasm is paying off. Since 1984, when Farrar, Straus & Giroux started to distribute Penzler's unlimited list, about 30 hardbacks a year, the Mysterious Press has become a major imprint. The Mysterious Press and Warner Books have just gone into partnership to publish 21 paperbacks this year and twice as many next year. Penzler can indulge himself publishing six to eight a year of his favorites in limited editions. With a slipcase and autographed, they sell for $45 instead of the regular $15.95.

Penzler's six-story, century-old Mystery Mansion has, in less than a decade, become almost as familiar to mystery fans as the House of Usher. When morning comes to the 19th-century graystone rowhouse at 129 W. 56th St., Penzler tumbles out of bed on the second floor and down a flight of stairs to his library. There he works surrounded by some 20,000 first-edition mystery books (most of them autographed) stacked two and three deep in floor-to-12-foot-ceiling bookcases that stretch wall-to-wall in the 20-by-30-foot room. Carolyn Hartman Penzler, Otto's wife and chief collaborator, hocked an engagement ring from her first marriage so they could build the bookcases and get Penzler's collection out of boxes at last.

The room would be a proper setting for the scenes where Nero Wolfe gathers all the suspects together to explain who done it. Mystery writer Bryan Brian ("Death Wish") Garfield has accused Penzler of buying the graystone in imitation of Wolfe, though it has no orchid-stocked greenhouse on top. The Mysterious Press and The Armchair Detective magazine, with a staff of five (they call themselves the United Otto Workers), are edited on the third floor.

* The Mysterious Bookshop, on the ground and first floors, looks so much like a mystery bookstore that the television pilot for "Murder Ink" was made in Penzler's premises. The shop's spiral staircase is very like the one in Dorothy L. Sayers' "Murder Must Advertise," in which a copywriter fell to his death. It leads up, if you don't suffer a similar fate, to the rarefied atmosphere of antiquarian mystery books. Mystery writers not only fill the bookshelves, they built them. Garfield said "the word had gone out" that he, Westlake and Justin Scott "were the crew to hire, so Otto hired us. We built the shelves all right, but the old house was so askew, Carolyn Penzler, the only master carpenter in the group, had to even up everything with cleverly designed molding." The writer-carpenters were paid off with a brass plaque proclaiming their achievement.

* "The day we opened the Mysterious Bookshop," Carolyn remembers, "we'd spent every penny we had. Otto opened the red piggy bank, our last savings, and blew everything on champagne, raw vegetables and dip for the opening. We made enough money that day to run us for the long months when we were teetering on the edge."

Before Michael Seidman became a consulting editor with The Mysterious Press, he worked in the shop on his Saturdays off from his job at another publishing house. Business was so slow, he and Otto would play chess in the long intervals between customers. Penzler sped things up by reapplying the party principle, giving book-signing parties for any author he admired whether Penzler published them or not. The shop caught on. Now it employs five people, dispensing imaginary murder-by-mail to the 5,000 names on a nationwide mailing list. True die-hards subscribe to the Crime Collectors Club to receive an autographed first-edition mystery a month. Three catalogues a year go out touting such esoterica as a stack of 60 of E. Phillips Oppenheim's 150 spy and mystery stories, or inviting readers to come and plow through a warehouse where Penzler stashed a huge mystery collection he'd bought.

* In the last 25 years, prices of mystery first editions have begun to approach James Bond's weapons bill. Agatha Christie's "The Murder of Roger Ackroyd" goes for $3,000. Dashiell Hammett's "The Maltese Falcon" costs about the same. Wilkie Collins "The Moonstone" would bring $10,000. In the unlikely event you were fortunate enough to have a first edition of Edgar Allan Poe's "The Murders in the Rue Morgue," you could count on at least $75,000.

Penzler has been the blunt instrument for raising prices of first editions of contemporary writers. The first two books by Dick Francis bring $500 and $800. An early Ross Thomas (the best selling contemporary author in the house) brings $100. The rare books account to about a third of the shop's profits.

* Penzler hasn't always presided over what his wife calls the "Ottoman Empire." Before he was married, he published a few mystery titles a year from a Bronx apartment so small he was too embarrassed to hire a secretary.

"I started selling 10 shares to my friends at $2,500 each -- I kept 50 percent of the shares. I priced the books at $10 so I could figure the discounts. I wrote the copy, edited the manuscripts, hired the artist, the printer. My brother came over once a week and helped me pack and ship. I didn't take returns, I sold by direct mail, 3,000 to 5,000 books." He supported himself by writing free-lance and playing poker with mystery writers. After they were married, Carolyn Penzler was the art department, doing covers and book design. Now she concentrates on a new line, a fancy edition with a stamped cover and an illustration pasted on. She paints portraits of the writers for the "The Mysterious Press Library of Limited Editions."

In between, she says, "I'm the building's super."

The key to Penzler "is his enthusiasm," says Thomas (whose novels sell faster in the bookshop than any other contemporary writer). "He elevates the suspense and mystery story to its deserved level of respectability."

Lawrence Block, guest of honor at many of the book parties, said Penzler made his name with writers because "he takes mysteries seriously. And everyone respects his encyclopedic knowledge of them."

"Taking it seriously" for Penzler means taking chances. Last October Penzler Books published Mcdonald's new nonmystery novel, "Safekeeping," about escapades of an English duke's 8-year-old son loose in New York. The book had been rejected by publishers for 12 years.

"I offered Mcdonald a $10 advance, a guaranteed print run and an advertising budget," Penzler said. "In effect, he shared the risk." The reviews of "Safekeeping" have been splendid.