Mini-epic couched in a riddle; Freddy's Book, by John Gardner. New York: Knopf. $10.

Freddy, the eponymous author of "Freddy's Book," is an 8-foot-tall giant of a boy, grotesquely overweight, monkishly reclusive, bookishly self-protective. Freddy is 16, a "monster," "a rhinoceros," "a small elephant," who hides in the upper rooms of his father's huge Victorian mansion, not far from Madison, Wis.

Father is a world-famous historian and scholar. Son is an unknown historical novelist. Father's specialty is Scandinavia. Freddy's novel concerns a critical moment in the history of 16th-century Sweden.

Socked in to his father's library like an airport in a snowstorm, Freddy has invented a tale, half history, half metaphysics, that involves Sweden's liberation from the Danes. It follows the evolution of Gustav Vasa from simple peasant to complex king. It shows his adviser, Lars-Goren, who just happens to be 8 feet tall, like Freddy, pitted against Hans Brask, a crafty Bishop, who might, if need be, "set the murderous Machiavel to school."

Also included in the cast of "Freddy's Book" are a Swedish pirate, a German general, a Lapland magician, assorted reindeer, and a mysterious saurian stranger who may or may not exist.

"Freddy's Book" has the makings of a Swedish national epic. Its larger-than-life hero, Gustav Vasa, a contemporary of Henry VIII and Ivan the Terrible, drives out the Danes, invents the use of printed leaflets as propaganda, circumvents various diabolic plots to overthrow him, and forges Swedish nationhood.

For opponent, King Gustav has a figure by now familiar to Gardner fans from as far back as "Grendel," a "man cut off from heaven by boredom and despair," the "dry as a spider" Bishop Brask. Negation in human form. In the Ingmar Bergman chess game that plays itself out down Gardner-Freddy's corridors of power, King takes Bishop. White to win, spider to lose. But only after some rather heavy metaphysical night-moves that reveal the Bishop's acedie,m a complicated state with its roots in the Bishop's discovery that "the literal world was no adequate metaphor."

"Freddy's Book" is jacket-styled a novel within a novel. It's not. It's an epic introduced by a novella. But why? Why does Gardner consign this book to Freddy? Could a 16-year-old recluse write this alternately portentous and politically savvy mini-epic? Not much. Is Gardner doing it for the sake of some game of parallels within and without the epic-cum-novella? Who will care?

And what becomes of the boy-giant turned novelist? Will he come out of the garret? Will wispy, waspish scholar-father approve of monster son's masterly effort? We'll never know.

And since Freddy's book is made to seem vastly more interesting than Freddy himself, we are probably not supposed to ask.