Havana, 1957. On the same day that the Mafia capo Umberto Anastasia is assassinated in a barber's chair in New York, a hippopotamus escapes from the Havana zoo and is shot and killed by its pursuers. Assigned to cover the zoo story, Joaquín Porrata, a young Cuban journalist, instead finds himself embroiled in the mysterious connections between the hippo's death and the mobster's when a secretive zookeeper whispers to him that he "knows too much." In exchange for a promise to introduce the keeper to his idol, the film star George Raft, now the host of the Capri Casino, Joaquín gets information that ensnares him in an ever-thickening plot of murder, mobsters, and, finally, love.   The love story is, of course, another mystery. Told by Yolanda, a beautiful ex-circus performer now working for the famed cabaret San Souci, it interleaves through Joaquín's underworld investigations, eventually revealing a family secret deeper even than Havana's brilliantly evoked enigmas.   In Dancing to "Almendra , " Mayra Montero has created an ardent and thrilling tale of innocence lost, of Havana's secret world that is "the basis for the clamor of the city," and of the end of a violent era of fantastic characters and extravagant crimes. Based on the true history of a bewitching city and its denizens, Almendra is the latest "triumph" ( Library Journal ) from one of Latin America's most impassioned and intoxicating voices.