Biography as Expiation : FORGOTTEN LIFE,<i> by Brian Aldiss</i> ,<i> (Atheneum: $19.95; 328 pp.) </i>

At least once in their lives many novelists who have been classified as genre writers will forsake their special preserves of crime, espionage, fantasy or science fiction to write, with varying degrees of success, what is called the straight, conventional or even serious novel.

John P. Marquand, for instance, succeeded admirably after abandoning the insufferable Mr. Moto. John le Carre, on the other hand, stumbled when he deserted the circus to write an almost unreadable romantic novel entitled, “The Naive and Sentimental Lover.”

The well-regarded British science fiction writer, Brian Aldiss, has now attempted the transition with a very serious, even somber, novel about two brothers, Joseph and Clement Winter, who were born in England 12 years apart, but separated throughout their lives by far more than just their ages.

When the novel begins, Joseph, the elder brother, is two months dead. The younger brother, Clement, is an analytical psychologist and fellow at Carisbrooke College, which is, he assures us, “less conventional than older Oxford College.” Clement is also married to Sheila, an enormously popular writer of fantasy novels that sell in the millions of hardback copies, at least in the United States, if not in the United Kingdom, thus enabling author Aldiss to indulge in some fantasy of his own.

It’s while they are on a promotional tour in the United States that Clement discovers his wife is having an affair with none other than her American editor. Enraged, he demands that she end it. But Sheila replies, “I’m enjoying it too much to stop,” a response that Clement, the surprised cuckold, later describes to his own analyst as the most powerful line Sheila will ever write or utter because, “It just destroyed me.”

Clement is still in this state of domestic destruction when they return to Oxford where he must not only settle his dead brother’s meager estate, but also determine how best to expiate the guilt he feels because of their long estrangement.

Expiation, Clement decides, might lie in a biography of the dead Joseph, a feckless adventurer, who fell in love with the Far East and spent much of his life there, never marrying, but enjoying the charms of literally countless women right up until the day he died of a heart attack at 62.

Some letters and a journal written by Joseph turn up. And it is through these that we are given the dead brother’s own fascinating story of how he served in England’s “Forgotten Army,” which protected India; fought in Burma and, just after World War II ended, was shipped to Indonesia in the faint hope that it might preserve a Dutch colony.

It is in this Dutch colony, teetering on insurrection, that Joseph falls in love with an attractive Chinese woman whom he promises to see safely to Singapore. Instead, he is forced to abandon her and spends much of the rest of his life roaming the Far East; pursuing his countless women and writing little-read books about obscure sultans, caliphs and paramount rulers whom nobody cares about.

Aldiss also uses flashback upon flashback to reveal the brothers’ rather horrible childhood and adolescence. Joseph, the elder brother, was born in the 1920s and grew up during the Depression and World War II. Clement, 12 years younger, was born just before the war and grew up in the bleak austerity that followed it. All this supplied their economic deprivation. Their emotional deprivation was furnished by their awful parents.

But it’s the older brother, Joseph, the adventurer, who suffers most from parental neglect, which, he feels, has helped destroy his life. Yet despite all his fretting, mostly over why mummy didn’t love him, the peripatetic Joseph still emerges as a far more sympathetic character than the stodgy, younger Clement. But then the picaresque character nearly always triumphs over the cuckold.

There is a certain amount of psycho-babble in “Forgotten Life.” Feelings are untouched. Relationships are plumbed. Childhood nightmares are regularly revisited. But this is to be expected when the narrator is a psychological analyst whose marriage is steadily disintegrating.

Aldiss draws a vivid, if unflattering, portrait of Sheila, the highly successful fantasy novelist, whose fictional heroine, “Green Mouth,” is inspired by a childhood doll that is still lugged around as a kind of talisman. He also introduces us to fantasy fiction groupies, who apparently are a distinct breed.

Aldiss obviously is much more than a competent writer. He has a clear style and a fine eye that he uses to create characters who, if not always likable, are certainly memorable. “Forgotten Life” is a superior, intricately plotted, sometimes irritating novel, which proves yet again, I think, that the cobbler need not always stick to his last.