THOSE who associate Miami Beach with wrinkled retirees will find their jaws dropping when they read Steven Gaines' new exposé, "Fool's Paradise," out Jan. 27 from Crown. The book details the city's hedonistic decadence, which includes how threesomes have become the rage, how the population of bisexual women is exploding and how easy it is to score drugs - "You can buy a tab of Ecstasy as easy as a pack of gum," Gaines writes. Among the bigger-than-life characters he chronicles are Thomas Kramer, the kinky German playboy whose $60 million Star Island estate has video cameras in every room. He also bares new details on the seamy story of Chris Paciello, the mob-linked nightclub czar who dated Madonna. "Miami Beach is a rich city on a little sandbar whose primary business is sex," Gaines says. "Miami Beach is all about a 'dirty weekend' and mindless good fun."