BOOK REVIEW : A Mystery That Isn’t: Hinging on a Gaffe : TELLING ONLY LIES <i> By Jessica Mann</i> ; Carroll & Graf $19.95, 256 pages

Try this on: A mystery writer writes about a mystery writer who no longer wants to write mysteries but writes one anyway.

Perpetrator of this tangle is Jessica Mann, author of such tidy if bloodless detective yarns as “The Eighth Deadly Sin” and “Faith, Hope and Homicide.” Her alter ego here is Anne Medlicott, “early 50s, married, prosperous, still quite good-looking,” whose reviewers often ask why she doesn’t write a “real book . . . to let it all hang out.” Anne/Jessica is tempted but loath to “perform one of those autobiographical stripteases.”

While still contemplating a sortie from the sanctuary of “intelligent, dispassionate, elegant” whodunits, Anne agrees to appear on a British TV round table with frenzied painter Perdita Whitchurch and Sir Hans Hahn, a distinguished historian. Hahn, a refugee from World War II Germany, brandishes a list of incipient Quislings--Britons on whom the Nazis could count to help them if England were occupied.

Anne peeks at the list, whereupon we are asked to believe that this urbane, sophisticated writer blurts, on camera, “Goodness, I’ve just noticed. Look, Perdita, your father’s name is on the list!”

It is one of a very few false steps taken by Mann, but since the entire book hinges on Anne’s gaffe, it’s a whopper.

Perdita’s father, Julian, now Viscount Blanchminster, sues for libel. Anne refuses to apologize; she’s half-Jewish. Partly to fend off the suit, partly from innate curiosity and a chance to write her “good book,” partly because her own family is implicated, Anne sets out to trace Julian’s putative involvement with the Nazis.

The chase kick-starts in Berlin in the infamous Olympic year of 1936. Germany’s between-war decadence is brilliantly evoked through the eyes of Celia Roget, a doughty young British woman.

As an official in the British Embassy (she writes to Anne), Celia was perfectly placed to report on the two prominent families frequented by Julian. On the far right are the Von Beowulfs: Baron Hubertus, the archetypal Aryan, dueling scars and all; Baroness Marianne, gorgeous and athletic, frequently found--with Julian--at the balls of Monaco, the ski slopes of Garmisch, the regattas at Cowes.

On the left are the Silberschmidts: the older generation gracious, generous, Jewish; the younger generation either defiant (Michael) or complaisant (Lotte). In the middle is Julian, who was at Oxford with Michael.

Celia, who knew the Silberschmidts, pays a visit in 1936. The chase heats up; the book turns cold. A neighbor says, “They came for them yesterday.”

Some of the least likely Britons have infiltrated high German circles and thus are able to set up a hazardous rescue pipeline.

Although speaking with Jews, let alone sleeping with them, is proscribed by the Nazis, relationships become entangled. Among the Von Beowulfs, the Silberschmidts and the Whitchurches, there are permutations and combinations, compounded by rampant bisexuality. Jewish blood blends with Aryan. A baby is born, deserted, adopted.

The ensuing war effaces almost all clues as to Julian Whitchurch’s involvement.

Julian, Viscount Blanchminster, offstage protagonist, dies of a stroke late in the book--news of which has no bearing on the outcome. Except this from Anne: “I look at the dying man and no longer want to know the truth about him.” Now she tells us.