Marco Polo, If You Can, by William F. Buckley Jr. (Avon, $3.50). Blackford Oakes, no longer with the CIA, suddenly finds himself reinstated for a doomed high-flying spy mission. Buckley, as always, entertains us with wit and a well-paced yarn about how a sly spy survives.

The Infinite Passion of Expectation, by Gina Berriault (North Point, $12.50). This collection of 25 stories, from a writing career of as many years, includes fiction first published in such diverse places as Esquire, Ploughshares and Mademoiselle. The title story is the exquisitely subtle study of a young woman who temporarily moves into the house of her aging psychiatrist.

Up Above the World, by Paul Bowles (Ecco, $7.95); The Orchard Keeper, by Cormac McCarthy (Ecco, $6.95).Two novels in The Ecco Press' Neglected Books of the 20th Century series. The Bowles novel, published in 1966, is about an American couple touring in a Central American capital; they come to learn that things there are not as they seem. The McCarthy novel, published in 1965, set in a small Tennessee town between the wars, involves three people, two men and a boy, who are estranged from the ways of the modern world.