Compelling spring reading; Spy hero's new escapades Marco Polo, If You Can, by William F. Buckley. New York: Avon. 262 pp. $3.50.

Blackford Oakes, the derring-do spy hero of this and three earlier novels, is re-recruited into the fold for a special assignment. ''Blackie'' dashes about the world in pursuit of a ''mole'' who has penetrated the National Security Council. It is giving little away to say that Oakes ends up crash-landing a U-2 near the Sino-Soviet border.

Oakes's escapades are interspersed with appearances by actuN-Oical figures: Khrushchev, Eisenhower, Dean Acheson. This use of imaginary conver3atioVs among the high and mighty makes Buckley's book different from run-of-the-mill thrillers; he is perhaps the only American spy author who uses domestic politics to provide the real intrigue. The spy genre benefits richly from his use of domestic politics.