The Hydra Head, by Carlos Fuentes, translated from Spanish by Margaret Sayers Peden. New York: Farrar, Straus & Giroux. $6.95.

Beneath the surface of this intricate spy thriller lies a social and political commentary laced with piercing insight by one of Mexico's leading novelists. The plot involves a bourgeois bureaucrat drawn into a newly formed espionage ring. His task: to prevent the theft of documents detailing the size of Mexico's oil reserves. Fuentes describes the national and international forces grappling for control of Mexican oil today.