DONALD E. WESTLAKE, the rakish comic spirit of American crime fiction, is back with the Dortmunder gang, that merry band of heistmen whose best-laid plans -- and crimes -- always seem to be undone by unforeseen twists of fate.

Good Behavior (Mysterious Press, $15.95) is the sixth Dortmunder caper. It is a choice Westlakean comedy of crime, with oddball characters, witty word play, and a clever scheme for a joint burglary-rescue mission that runs into hilarious complications.

In their first appearance in The Hot Rock (1970) Dortmunder's hapless burglars had to break into a jail and an insane asylum in an attempt to retrieve an emerald swallowed by a colleague moments before his arrest. In Bank Shot, the heist artists pulled off a brilliant ploy by stealing an entire bank housed in a mobile trailer, only to have it careen from their grasp.

Now, in Good Behavior, fortune finally deigns to smile on John Dortmunder, May, Andy Kelp, Stan Murch, and Tiny Bulcher (whose bulk inspires a Japanese businessman to mutter ''Godzilla'' when the two share an elevator).

Not that any one would guess that things are looking up for the Dortmunder gang when the story opens. A bungled burglary ends with Dortmunder crashing through a skylight into the rafters of a downtown Manhattan convent. He has to play a game of charades to try to explain his unorthodox entrance to the nuns, who are under a vow of silence (except for two hours on Thursdays). The members of the silent sisterhood -- with such names as Mary Forcible, Mary Vigor, and Mary Capable -- view Dortmunder as an answer to their prayers, for they need a man with his particular talents to rescue a kidnaped sister. She is Mary Grace, daughter of a tycoon who has called in a deprogrammer and imprisoned her in a penthouse perched atop a high-security tower in midtown Manhattan.

The nuns strike a bargain with Dortmunder: They will not call the police if he helps them free Sister Mary Grace. So there is Dortmunder trying to figure out a way to steal a nun.

Never one to miss a little profitable business on the side, Dortmunder comes up with a scheme to hide in the empty skyscraper over the weekend and leisurely rob the offices of the jewel and gem importers.

When it comes to sneaky action, Sister Mary Grace is no slouch herself. She manages to smuggle out a list of tenants and their security arrangements under the voluminous folds of the skirt of her Guatemalan maid. When a stunned Dortmunder is handed the security guide, he offers up his thanks and tells the Mother Superior: "Let us prey."

But no Dortmunder escapade is without complications. The tycoon also plans to use the deserted skyscraper that same weekend for training maneuvers for the band of mercenaries recruited to unseat an uncooperative South American dictator. Dortmunder, on his burglary-rescue mission, soon is confronted by gun-toting guerrillas. The silent sisterhood has to mobilize to save its rescuer from both the mercenaries and the police.

In the midst of these antics, Westlake never allows his comedy to slide into silly farce or buffoonery. Dortmunder and his pals may be luckless losers but they are not inept bunglers. Their plans can run into snafus like those of any legitimate businessman.

In Good Behavior, Dortmunder and May get time off for their chivalric act of rescue and finally get to enjoy the loot of a successful caper.

Then and Now

IN HIS SECOND mystery, Bertie Benham, a daring, inventive writer, essays not only difficult split-narration but "then" and "now" views spanning two generations of a family.

It works brilliantly in Two Thyrdes (St. Martin's, $15.95). There may be a few moments of confusion in keeping track of the twin story threads separated by 35 years, but the device allows Denham to interweave the parallel tales, each equally compelling and reflecting the social climate of its time.

In 1944, Derry, Second Viscount Thyrde, joins six colleagues to celebrate a friend's birthday. That evening he stumbles on evidence that one of the party is a traitor. Before he can trace the identity of the turncoat, Derry is killed on a battlefield mission.

Thirty-five years later, Derry's son joins the survivors and sons of the World War II group in the annual reunion of the 1944 Club. Like his father, Derek finds the evening ends in betrayal. Someone hates the Thyrde family enough to frame Derek for heft and threaten to ruin his political caeer.

The stories of the two Thyrdes, father and son, are told in alternating "then" and "now" chapters. Lord Denham, a veteran member of the House of Lords and a privy councillor, received well-deserved applause for his mystery debut in The Man Who Lost His Shadow. Two Thyrdes is even better.

Alphabetical Murder

SUE GRAFTON's latest Kinsey Millhone outing, "C" Is For Corpse (Holt, Rinehart and Winston, $14.95), also will not disappoint fans of her top-notch series.

Kinsey, a thoroughly professional private investigator, is a clear-headed, independent, unsentimental gal not given to the introspective anguish that plagues so many of her colleagues, both male and female. She has been divorced twice (in the latest case, we learn that one marriage was to a jazz pianist who became hooked on drugs), lives in a 15-foot-square converted garage that simplifies life ("I like being alone, and I suspect my independence suits me better than it should").

Kinsey is a licensed private eye in San Teresa, California, north of Los Angeles, a terrain made familiar by Ross Macdonald.

In "C" Is For Corpse, Kinsey's client is a dead man. She met Bobby Callahan on Monday. He said some one was trying to kill him. By Thursday, he was dead. Callahan had survived one attempt on his life in what was ruled a car accident. It had left him brain-damaged, trying to remember the scrap of knowledge that prompted some one to want to kill him.

At the same time that Kinsey is tracking down Bobby's murderer, she is trying to save her 81-year-old landlord from the flirtatious wiles of a senior-citizen scam artist who has bedazzled him with her Southern-widow pose.

Grafton is a talented writer with an engaging breezy style and characters that are real and believable people. Fans of Kinsey Millhone will be happy to know that "D" Is For Deadbeat is well under way. There are 22 other characters in the alphabet.

Monastic Mayhem

WHEN the school nurse's body is found behind a doorleading into the monastery, it is clear that she has been murdered. For the Matron, "would never, voluntarily, have invaded the monks' privacy: still less would she have broken the strict rule that denies all women -- other than the reigning monarch, Consort, or Queen Mother -- entrance to a monastery of the Order of St John the Less."

Snares of the Enemy (Scribners, $13.95), a murder-in-the-monastery, is Pauline King's first mystery novel, one that affectionately evokes the atmosphere of an English monastic public school -- the sense of pride in long tradition, the pervasive role of religion in the daily routine, and the interplay of a closed community with its school boys, supervising monks, lay teachers, school matrons, games masters, village maids, and old-boy parents.

The death of the matron brings Inspector Evan Morgan, a Welsh Methodist to Ambelhurst, where he soon discovers that all is not peaceful and serene within the cloistered walls. There is staff bickering, and the cricket master has been playing games with the Irish and Spanish maids.

A second stabbing death occurs before the inspector, assisted by the shrewd monk-headmaster, identifies the disturbed murderer. The policeman defers to the abbot to lead the murderer to confession, both for spiritual cleansing and legal evidence. Unfortunately, this device comes off as an exercise in pop psychology combined with religious instruction.

Aside from this miscalculation, Snares of the Enemy is a debut with considerable promise. King, a literate writer with a wry touch to her kindly humor, sketches sharp portraits of the school community, including the guest-master, a kind of monastery hospitality hostess, who knows how to flatter the wife of an old boy into sending her son to Ambelhurst instead of her father's school.

Shamus in Trenton

I FRANKLY must confess that I don't know quite what to make of Kate Gallison's Unbalanced Accounts (Little, Brown, $14.95). Is it meant to be a parody of the hard-boiled detective novel? Or is it written as a quirky, black-comic private eye tale set in Trenton, New Jersey, with a shamus named Nick Magaracz ("rhymes with pots")?

Whatever the identity problem, Unbalanced Accounts is a slim first novel (131 pages) that is fitfully amusing and entertaining. There are some funny moments and lots of bright banter. But these are only flashes in a mystery that often strains painfully to be wildly comic and witty.

The private eye business hasn't been too brisk in Trenton with divorce-law reform, so Nick Magaracz decides to take a job in the State Bureau of Mental Rehabilitation, which is missing 375 checks that disappeared before they could be mailed. His wife's cousin Charlie, the office supervisor, hires Nick to go undercover as an accountant to trace the missing checks. You might call it a case of industrial espionage, or so Nick would like to believe, His investigation seems to consist largely of eavesdropping on the gossiping girls in the 12th-floor bookkeeping department.

The missing checks also have upset the manger of the Boardwalk View Rest Home for the Disturbed Elderly in Atlantic City, who has been appropriating his boarders' state allotments. His two loutish sons, frustrated by the bureaucracy's slowness to reissue the checks, take off for Trenton and commit three murders in an attempt to speed up the process.