Pick of the paperbacks; Freddy's Book, by John Gardner. New York: Ballantine Books. $2.95.

Sixteen-year-old Freddy is an 8-foot-tall giant of a boy, grotesquely overweight, monkishly reclusive, who hides in the upper rooms of his father's huge Victorian mansion.

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

Freddy has invented a tale that has the makings of a Swedish national epic. Half history, half metaphysics, it involves Sweden's liberation from the Danes. Its larger-than-life hero, Gustav Vasa, drives out the Danes, circumvents various diabolic p lots to overthrow him, and forges Swedish nationhood.