THE KARNAU TAPES.<i> By Marcel Beyer</i> .<i> Translated from the German by John Brownjohn</i> . <i> Harcourt Brace: 242 pp., $23</i>

Hitler, as far as we know, was nice to his mistress, Eva Braun, and apparently she was content to die with him in the Berlin bunker. And in his diary, Goebbels wrote of his six children: “I hear the sweet little voices that are dearer to me than anything else in the world. What a precious possession! May God keep it safe for me.” God had other plans, and Goebbels disposed of his “possession” with cyanide, reasoning that if you can’t take it with you, neither should you leave it behind.

The quote from Goebbels introduces Marcel Beyer’s coldly unsettling novel “The Karnau Tapes.” No doubt it will fail to reach the jury in the trial of accused Oklahoma City bomber Terry Nichols. It would unhinge the very assumptions underlying any defense that attempts to portray him, however genuinely, as a devoted family man.

Beyer, a young German writer, explores Nazi evil in terms not merely of its monsters’ banality but of their amiability. It is an evil--the present tense is appropriate because Beyer’s implications do not end with one stretch of history--that is all the more terrifying because it takes root in a patch of deformed virtues: loyalty, patriotism, efficiency, scientific experiment and even tenderness.

Beyer is not breaking new ground. There are shelves of books on the subject, including several studies of Hitler’s architect, the retrospectively self-cleaning Albert Speer. But Beyer sifts the ground and shapes it to startling effect. He has better judgment than to press us with the tender side of Goebbels, whom we glimpse only in flashes. Rather, the tenderness works as one bewildering strand in the lives of Goebbels’ six little children. Their splintering sense of reality, as the war nears its end, is one of the book’s two central narratives.

They try to hold on to their image of a loving and powerful father, even as it is undermined by questions and, finally, by a terrible certainty. Related by Helga, 8 years old at the start and 12 at the end, doubts and certainty come in half-portions and at half-strength. Beyer never allows his adult outrage to falsify a child’s voice, subject as it is to wandering attention and the capacity to hold two contradictory emotions, trust and fear, at the same time.

Even at the start, when the war is going well and the Goebbels family enjoys country houses and a Berlin mansion--as well as servants, a private screening room and public applause--Helga’s voice shows tinges of unease. Their mother is periodically hospitalized for nervous prostration; their father is away on the job or, as the child discovers from an indiscreet glimpse through his office door, with a lover.

The children invent games. They act out an imaginary film and take turns playing censor. The oldest play Brownshirts, forcing the little ones, the Undesirables, to kneel and scrub the carpet with toothbrushes, while gently kicking them. Corruption coats their innocence like falling ash; they will not live long enough for it to corrode.

The war closes upon them. Refugees move in, the food turns meager, the servants leave. The mother spends her time weeping or in bed. The father grows grimmer, and one day Helga hears a broadcast in which he announces that defeat is inconceivable because, if it came, neither he nor his children could live. With bombs destroying Berlin, Helga begins to guess what is in store, a guess that approaches certainty when the family moves into Hitler’s labyrinthine bunker.

Solitary in her dread, she turns to a gentle and sympathetic family acquaintance: Karnau, the acoustic experimenter who installed the sound system for Goebbels’ prewar rallies, worked on a series of special wartime tasks and now is assigned to the bunker. His mission: to record Hitler’s final moments.

Karnau reassures Helga that she will be safe. He steals a chocolate bar for the children from Hitler’s personal stock. Seemingly, he is the only sane adult in the bunker madhouse. Yet in his technological zeal, he betrays Helga’s trust by hiding a microphone under her mattress. It will record her last words--”Is that you, Herr Karnau?”--in hopeful response to a noise at the door, which is about to open to their silent executioner. Who this is remains in doubt: probably the mother, just conceivably Karnau, unquestionably Hitler.

Karnau, in fact, is Beyer’s protagonist. He personifies the insane coexistence between human decency and monstrosity effected by a system whose enforcement of its single purpose excludes all other considerations. His story alternates with Helga’s until they converge in the bunker, after which her living voice gives way to the recording.

Not even in the most advanced robotics is the machine equivalent to life, because it is nothing but purpose. Yet here it has replaced life. So it has in Karnau: Beyer’s symbol of the scientist, the bureaucrat, the doctor and the butcher who avoided noticing many other things while concentrating on science, efficiency, medical research and cutting meat, inspiredly, even.

Certainly Karnau is inspired. From childhood he was fascinated with sounds. Wiring Goebbels’ outdoor speeches has been a fascinating technical challenge; sent to the Russian front to pick up radio traffic, he places front-line microphones at great risk to pick up the sighs, gurgles and whimpers of the dying.

He assists the Germanization program in occupied Alsace--French was forbidden--by doing the acoustical work. He is genuinely unhappy when a recording uncovers a French speaker in the Germanizing team, thus identifying him as a member of the Resistance. Furthermore, before a conference of linguists, he courageously denounces as “Brownshirt stupidity” the effort to eradicate a native language. You would have to do it surgically, he said, thinking to be ironic.

Lacking irony, the Gestapo enthusiastically places him with a team of doctors who dissect and transplant the voice boxes of prisoners. In a series of near-unbearable scenes, Karnau records the acoustics of the unsatisfactory results, along with the screams of the tortured victims. The screams trouble him; he worries about the damage they do the vocal cords.

As a novel, “The Karnau Tapes” doesn’t quite keep up with its mordant and troubling theme. There is a suggestive counterpoint between Karnau’s monstrous excursions and the narrowing circle in which Helga and her siblings are caught--precisely because of such excursions. The children’s scenes are lifelike and affecting; if they are less than tragic, it may be because they are so aptly arranged to carry Beyer’s ideas.

Karnau’s moments of decency, his thoughtful care of the children, for instance, are written without irony. They seem like the real thing. This, oddly, is a problem. On an intellectual level, it fits the theme of Nazism’s grotesque insertion of monstrousness into human natures that seem to have no special disposition toward evil. Fictionally, we may require something more in the way of either integration or embattled paradox. Beyer doesn’t manage it; as it is, Karnau remains a man of instructive but unassembled contradictions.