William Sleator, a writer for young people whose books pitted their heroes against aliens, ghouls and slimy things, not to mention the most malevolent rivals of all — siblings — died Wednesday in Bua Chet, Thailand. He was 66.

The cause had not been determined, his brother Daniel said Friday. He added that Sleator, who had struggled with alcoholism for many years, had been having seizures recently.

Working in a genre that straddled fantasy, science fiction, horror and suspense, Sleator wrote more than 30 books. Most were for young adults, though some were aimed at middle-grade readers. Critics praised his spare, stylish, often darkly comic prose; hurtling plots; and deliciously strange characters, among them a gasbag-like flying octopus.

Moody, psychologically probing and sometimes terrifying, Sleator’s work chronicled young people’s passage through all manner of dystopias. It was a fitting juxtaposition of age group and subject matter, for what, after all, is more dystopian than adolescence?

‘Interstellar Pig’

In confronting the grotesque, the menacing and the outright evil, Sleator’s protagonists simultaneously confront their own identities and their relationship to their families, especially to brothers and sisters.

His best-known novels include “Interstellar Pig” (1984), involving a youth who is drawn into an all-too-real role-playing game — here enters the octopus — in which the losers and their civilizations are destroyed, and “House of Stairs” (1974), about teenagers trapped in a malign behavioral experiment.

He was also known for “The Green Futures of Tycho” (1981), in which a boy travels forward in time and meets his adult self. The protagonist was named for Sleator’s youngest brother, Tycho; early on, he often co-opted family and friends as characters until, he later said, he had run out of friends in every sense.

Physiologist father; mother studied ADD

William Warner Sleator III was born on Feb. 13, 1945, in Havre de Grace, Md., and reared in University City, Mo., a St. Louis suburb. His father, William Jr., was a physiologist; his mother, Esther Kaplan Sleator, was a pediatrician who did early research on attention deficit disorder.

Billy, as he was known, grew up amid art, intellectual ferment and a laissez-faire approach to child rearing that would give helicopter parents the fantods. He captured the milieu in “Oddballs” (1993), an autobiographical volume centering on his life with his brothers and sister, Vicky.

“As teenagers, Vicky and I talked a lot about hating people,” he wrote. “At the dinner table, we would go on and on about all the popular kids we hated at high school. Dad, who has a very logical mind, sometimes cautioned us about this. ‘Don’t waste your hate on them,’ he would say. ‘Save it up for important people, like the president.’ We responded by quoting the famous line from ‘Medea’: ‘Loathing is endless. Hate is a bottomless cup; I pour and pour.’”