There are more things in heaven and earth -- this includes Philadelphia -- than are dreamt of in anyone's philosophy, unless you include the philosophy behind yet another book in the ongoing and well-publicized spate of de facto non-fiction, or de ficto non-fact, depending on your persuasion.

Yes, if you loved the green slime sliding down the walls in Prentice-Hall's "The Amityville Horror," and if you were one of the estimated 105,000 people who paid hardback prices for Lippincott's "In His Image: The Cloning of a Man," you'll be ready in April for the curious to overwhelm the spurious as Grosset & Dunlap brings out "The Philadelphia Experiment: Project Invisibility."

"I suppose it's the inevitable result of an industry that looks for spectacular ideas," says New York literary agent John Sterling, evoking thoughts of "Invasion of the Body Snatchers" when he adds: "It's like in Hollywood where all the big studios are making what used to be B movies, but this time with big budgets."

And, says one publisher of best-selling non-fiction: "Publishing is increasingly competitive. The valves get turned up: What are we going to do to pay the rent?"

In the latest turn of the valve, Grosset & Dunlap promises "concrete proof" and "solid evidence that in 1943, as an outgrowth of research designed for defense, Navy scientists found a way to make matter invisible."

Specifically, the book claims that a ship was made to vanish in the open sea, except for the imprint of the hull in the water -- rather like the footprints left in the snow by Claude Rains in "The Invisible Man." Best of all, the Navy, says the publicity, "succeeded in making a ship vanish from the Philadelphia Navy Yard and within minutes reappear in Norfolk, Virginia, before being whisked back to Philadelphia."

The publisher plans a first printing of 25,000 copies. Foreign rights already are reported to have earned over $100,000. The New York Post, Variety, Publisher's Weekly, The Hollywood Reporter and ABC's "Good Morning America" have all spread the word.

"The endeavour to wrap the mysterious in the tone of sober reportage represents an attempt to have it both ways," explains social and literary critic Benjamin DeMott, at Amherst.

"On one side, you have the manner of facts. On the other, you have a mystery. These books attempt to deal with the mystery in a straightforward way, so that even if you're educated as a skeptic, as we all have been since the 19th century, you don't feel like an oddball reading a book like this."

Then again, fact and fiction, once clear and distinct categories, have been run through the Cuisinart of contemporary confusion until the resulting melange has become a genre in itself. In "Ragtime," a novel, E. L. Doctorow introduced such real-life characters as J. P. Morgan and Emma Goldman, not just in passing, but with dialogue and characterization. Max Apple's "The Oranging of America" gives us witty, fictional biographies of figures such as Howard Johnson and Walt Disney. Science-fiction buffs love to tell how the FBI grilled one early-'40s writer who described the building of an atom bomb.

As if to sum up the whole dilemma, occultists have a standard retort to all who challenge the factual veracity of Carlos Castaneda's accounts of seeking enlightenment with Mexican shaman Don Juan: "What difference does it make?"

If the style of "The Philadelphia Experiment" relies on scientific syntax, the content verges on pure twilight zone. The result, once the reader is recondiled to the blurring of fact and fancy, is often pure comedy.

The publisher's "solid evidence," the sine qua non of the "concrete proof" in fact, is a number of letters written by one Carlos Miguel Allende, aka Carl Allen, from an untraceable address in New Kensington, Pa., and sometimes on the stationery of the Turner Hotel in Gainesville, Tex. All of them are couched in an eldritch clamor of doomsaying and eccentric punctuation, a mix of electronic theory and political conspiracy.

He writes: "We today Would NOT exist of it We did exist, our present Geopolitical situation would not have the very time-bombish, ticking off towards Destruction, atmosphere that Now exists. Alright, Alright! The 'result' was complete invisibility of a ship, Destroyer type, and all of its crew, While at Sea. (Oct. 1943)..... There are only a few of the original Expierimental D-E's Crew Left by Now, Sir. Most went insane, one just walked 'throo' His quarters WALL in sight of His Wife & Child & 2 other crew members (WAS NEVER SEEN AGAIN)."

Perhaps well aware that his letters might get tossed in the crank mail bin, the writer signed one: "Very Disrespecfully yours, Carl M. Allen.

Even more annoying, or perhaps convenient, Allen/Allende is missing. All other witnesses, be they second-hand, circumstantial or material, are either missing, dead, or refusing to let their names be used.

And while the Amityville and cloning books at least had new stories to offer, this one has to concede that the ship rumor has "been mentioned in books and articles by at least a dozen writers and researchers of the unexplained in the course of the past two decades."

A science-fiction novel, "Thin Air," used some of the same material in its description of a vanishing ship and force-field tests.

The Pentagon agrees. "The genesis of the Philadelphia Experiment myth dates back to 1955 with the publication of 'The Case for UFOs' by the late Dr. Morris K. Jessup," says a Navy form letter which is sent to all inquirers about the incident.

"Over the years we have received innumerable queries about the socalled Philadelphia Experiment," the letter reads. "The frequency of these queries predictably intensifies each time the experiment is mentioned by the popular press, often in a sciencefiction book."

But arguing with true believers in this field invites a barrage of semantic grenades such as "strange coincidence" and "isn't it curious that..." or the wonderful last line of the book: "If the Philadelphia Experiment never happened as described, what actually did happen in a high-security area of the Philadelphia Navy Yard in October, 1943?"

To his credit, author William Moore, a Herman, Minn., high-school teacher and flying saucer investigator, claims that the line actually was written by Charles Berlitz, whom he wrote the book "in consultation with," according to the cover.

To his credit -- especially financial -- Berlitz has been mining the mother lode of weirdness for years with books such as "The Bermuda Triangle," "Mysteries of Atlantis," and "Mysteries From Forgotten Worlds."

"The traditions of this sort of thing go back a long ways," says Leslie Fiedler, literary critic and professor at the University of Buffalo. "It's publishing business as usual. For instance, 200 years ago, because of the great distrust of the new medium of fiction, especially by the church, it was often presented as fact. Readers didn't know whether 'Robinson Crusoe' was true or not. Poe loved playing games with his readers.

"Finally, there's a show business tradition. One 18th-century English-woman made a living selling pamphlets on how she'd given birth to six rabbits. P.T. Barnum's sideshow attactions all sold pamphlets on themselves. There's always been all kinds of equivocation about fact and fiction: Look at the Carlos Castaneda books."

Ahhh, the speculation. After a hundred pages or so, the reader gets the hang of it -- all those hints of knowledge too dangerous to be released, that Einstein himself was in on it, the old tricks such as Moore claiming that he actually set out to disprove the whole thing, but as the facts mounted up...

Until, to the ultimate sidewalk cabalist, the whole thing no doubt becomes clear: Any book so clumsy must be a ruse, a dodge to throw us off the trail of Something Really Big: Judge Crater? The second gunman in Dealey Plaza? The eggplant that ate Chicago?

After all, if this book isn't true, what actually did happen in a highsecurity area of Grosset & Dunlap in 1978?