There are no mentions of Rosebud, no allusions to the auteur's infamous girth. The Orson Welles in Kaplow's inventive novel is a 22-year-old genius who's directing and starring in ''Julius Caesar'' at Broadway's Mercury Theatre. He's also the man who changes -- for better and worse -- the life of 17-year-old Richard, a suburban New Jerseyite living vicariously through luminaries like Noel Coward (though Coward wouldn't have been caught dead drinking Black Crow beer). In a too-good-to-be-true tale, Richard winds up playing Lucius to Welles' Brutus and, in the span of 269 breezy pages, falls in love, has his heart broken, sees his showbiz dreams crushed, and -- beautifully, almost imperceptibly -- becomes a man. And his happy ending is only the beginning.