Where the Incorrigible Meets the Unpronounceable : DON’T ASK: <i> By Donald E. Westlake</i> : <i> (Mysterious Press: $18.95; 327 pp.) </i>

John Dortmunder is no prince of thieves. Gloomy, badly dressed and rarely articulate, he is not in danger of being mistaken for Raffles or even Arsene Lupin. In fact, Dortmunder isn’tmuch of a bandit at all some days. Like the time he hijacked a truckload of fresh fish and mistakenly turned off the air-conditioning system before a steamy four-hour drive on the Brooklyn-Queens Expressway. Don’t ask about the stink.

John Dortmunder is, as “Don’t Ask,” the eighth novel to feature his dubious exploits proves, the funniest practitioner of a nominally serious business. Whatever can go wrong in the man’s elaborate attempts at larceny invariably does, and in the most amusing and unexpected ways possible.

Dortmunder is the creation of Donald E. Westlake, a caper novelist both prolific and underappreciated. Despite having written somewhere close to 100 novels under his own name and a variety of pseudonyms, plus the screenplays for “The Grifters,” “The Stepfather” and who knows what else, Westlake never seems to get quite as much public acclaim as his grateful fans think he deserves.

Aside from the Richard Stark novels, involving a laconic torpedo named Parker (as in “Parker steals, Parker kills, it’s a living”), Dortmunder is Westlake’s most durable character. Several of his previous appearances, such as “Bank Shot” and “The Hot Rock,” have been filmed, and this most uncertain of thieves has been played by everyone from Robert Redford to George C. Scott.

None of this recognition, fortunately, has gone to our hero’s head. He still does business under the same black cloud with the same unpredictable associates: obsessive driver Stan Murch; the nimble Andy Kelp, who picks locks instead of using doorbells, and the bigger-than-a-breadbox Tiny Bulcher, conservatively estimated as “the size of a minor Alp.”

It is Tiny who gets this latest exploit under way. For who should show up in New York but his long-lost cousin from the old homeland, the unpronounceable Grijik Krugnk. Grijik hails from Tsergovia, the arch-rival of another tiny Carpathian entity called Votskojek. Both countries want to get into the United Nations, but the place is big enough for only one, and that, for reasons way too complicated to explain, will be the country that can prove possession of the most revered of religious relics, the femur of St. Ferghana.

This bone is the only trace left of a 16-year-old believer who was killed and eaten by her less-than-orthodox family in the year 1217. It now reposes in the Votskojek Embassy, in reality a former tramp freighter gloomily anchored in the East River. Grijik, in his best broken English, appeals to Dortmunder and company to lift it, thereby “doing a wonderful thing for a little country that never hurt nobody.”

Never mind that Tsergovia is a country so poor its only export is rock, and that Grijik wants to pay Dortmunder in draffs, the local currency that is going for 2,650 to the penny. Or that the motto on Dortmunder’s family crest is Quid lucrum istic mihi est? , more commonly known as “What’s in it for me?” It seems fated that he and his team get involved, and that everyone within striking distance will live to regret it.

One of the keys to the Dortmunder novels’ success is that Westlake has the ability to write tough as well as easy, that he knows how to plot a complex crime as well if not better than his straight-faced brethren. And despite his unfortunate tendency to bungle early and often, Dortmunder is a more or less capable felon, and the comic-opera surface of his adventures conceals the elaborate strength of a tiptop precision instrument.

“Don’t Ask,” like many of Dortmunder’s tales, involves not one crime but two, the one that Dortmunder thinks he is doing and a later one undertaken to, uh, clean up a few loose ends. The stray elements this time involve not only Hradec Kralowc, Votskojek’s party-animal ambassador, but an aristocratic fence, a cranky archbishop, a doctor unnerving enough to frighten the South Bronx, and hotel magnate Harry Hochman, unhappy proprietor of the Happy Hour Inns.

As fastidiously as these novels are plotted, it is the journey, not the destination, that is the pleasure of Dortmunder’s exploits. No one’s touch is as quixotically cockeyed as Westlake’s, no one can keep you chuckling as continuously or make literary amusement look easier. “Don’t Ask” is not brilliant or dazzling, it is merely wonderful, and if you are asking for anything more, you’ve got a lot to learn.