BOOK REVIEW : 2 More Good Yarns by a Savvy Storyteller : RUMPOLE ON TRIAL, <i> by John Mortimer</i> ; Viking; $21; 243 pages : DUNSTER, <i> by John Mortimer</i> ; Viking; $21; 296 pages

Rumpole of the Bailey is back, and he’s brought along an intriguing but distinctly unsettling chap named Dunster. The two characters are evidence of the remarkable versatility of their creator, barrister-turned-novelist John Mortimer.

Rumpole, of course, is Mortimer’s most celebrated literary invention, and “Rumpole on Trial” is the latest outing for the gruff but hugely entertaining criminal attorney who is now personified on PBS by the figure of Leo McKern.

“Dunster”--another new book from John Mortimer--is a very different story indeed, a thriller set in contemporary London, but set in motion by an act of treachery and violence that took place in the High Appennines during the Second World War. The same seasoned storyteller is clearly at work in both books, and each book is a quick and compelling read, but “Dunster” is the more ambitious and accomplished of the two.

“Rumpole on Trial,” the ninth in the series, consists of seven new cases in which the redoubtable crusader for justice and common sense prevails once again over the stupidity and cupidity of the bench, the vanity and ineptitude of the bar, the mendacity of his own clients and the general decline of Western civilization. And, once again, Rumpole displays a wry and worldly-wise sense of humor that enlivens what might otherwise be seen as a rather sorry way of making a living.

“A world without evil might possibly be a damned dull world--or an undamned dull world, perhaps I should say,” says the “old buffer,” “and it would certainly be a world which would leave Rumpole without an occupation.”

Rumpole concerns himself with an apparent kiddie cult of devil worshipers, an enchanting but deadly virtuoso, an English lord with more than one skeleton rattling around in the castle, and miscellaneous other malefactors. At the same time--and much to the reader’s amusement--he is engaged in a running contest of wit and will with his conspiratorial office-mates and his formidable wife, whom he calls “old thing” to her face and “She Who Must Be Obeyed” in asides to the reader.

“You’d rather defend a murderer,” sniffs Hilda Rumpole, summing up Rumpole’s hierarchy of values, “than dance with your wife.”

“Dunster” is the story of two childhood friends, Dick Dunster and Philip Progmire. They encounter each other in middle age when they are assigned to work together on a television documentary on the subject of war crimes--including one long-suppressed atrocity that may have been the work of British commandos rather than the SS. What really happened, we are invited to speculate, on the night in 1944 when someone dynamited the church of Saint Magdalena in Tears in the High Appennines and sent the worshipers to their deaths?

At stake in the mystery is not only the verdict in a libel trial but, more crucially, the reputation of a knighted war hero who happens to be Progmire’s boss, the love of Progmire’s family, and--in the end--all of the comfortable certainties of Progmire’s moral universe.

Dunster is a memorable and engaging character--a man afflicted with what Progmire calls “an unreasonable addiction to what he felt to be the truth.” He’s a not-so-merry prankster who delights in challenging the conventional wisdom wherever he finds it. Even as a child, when Dunster and Progmire first meet each other in a London school, Dunster is a goad and an iconoclast.

“St. George was a Palestinian pirate and a brothel owner,” says Dunster of the patron saint of England and the namesake of the school. “I thought everyone knew that.”

Dunster haunts the otherwise comfortable Progmire, first at St. George, then at Oxford, through the years of courtship and career-building in the entertainment industry, and even on the day of Progmire’s wedding. Sometimes the trickster, sometimes the confidant, but ever the truth-teller, Dunster’s friendship is a kind of affliction. And there’s something dangerous, something ominous, that we begin to glimpse beneath his high spirits and his practical jokes.

“He’s a disaster area,” Progmire says of Dunster. “He’s a minefield. He invites every sort of catastrophe. He’s accident-prone and his condition is highly contagious.”

By the time Dunster and Progmire cross paths again in adulthood, the confrontation is no laughing matter at all. Dunster has staked a claim on Progmire’s wife and family. He has accused Progmire’s corporate mentor at Megapolis Television of mass murder, and he has tried to enlist Progmire as a fellow accuser. The climax is a libel trial in which the accuser, Dunster, and the accused, Sir Crispin Bellhanger, are weighed--and, in a real sense, both are found wanting.

In both “Rumpole” and “Dunster,” Mortimer succeeds in evoking the peculiar blend of energy, elegance and ennui in contemporary England with the same knowing sensibilities that one finds, for example, in the early novels of Len Deighton. And Mortimer does nearly as good a job at creating and sustaining an atmosphere of suspense and peril amid the seemingly bland setting of country estates and London office blocks, “pink and white Edwardian villas” and “warehouses tarted up into flats.”

Each of the books may be approached as nothing more than a good yarn by a savvy storyteller with a sturdy sense of humor, but they are hardly interchangeable. “Rumpole on Trial” is almost quaint, and--despite the updated setting--I was reminded of the pleasures of reading a book of Sherlock Holmes stories. “Dunster,” by contrast, turns up the moral heat on the reader, and Mortimer--rather like Dunster himself--insists on asking us the hard questions and confronting us with the ugly truth.