''They lay in the brush, in stickers and thorns, amid snakes and scorpions, under a fiery sun on an airless day. They lay as intimately as inverted lovers, though no sexual tension was felt by either. There was too much fear for that.'' If you'll believe it (and you will), that's a young Fidel Castro, cuddled up with a top Soviet spy and hiding from a torture-happy band of Batista's soldiers. Hunter's latest romp plops his taciturn, sharpshooting Arkansas war hero, Earl Swagger, into sweaty Cuba circa 1953, which bursts with CIA spooks, Mob mooks, and ancient prostitutes -- as well as the author's trademark ripsnorting gunfights. Cameos by Mob king Meyer Lansky and expat Ernest Hemingway (here a blowhard barfly) only add to ''Havana'''s juicy, jolty fun.