A PI performs some fancy footwork to protect Fred Astaire as &;Edgar winner Kaminsky effortlessly choreographs Hollywood history . . . and dirty doings&; (Publishers Weekly).   Sometimes fools must step in where Fred Astaire fears to tap.   Luna Martin, the moll of a well-known Los Angeles gangster nicknamed &;Fingers&; (because he likes to cut them off), has demanded dance lessons from Hollywood&;s finest hoofer&;and whatever Luna wants, Luna gets.   To sidestep the flirtations of the lead-footed lady, Astaire hires private investigator Toby Peters to pose as a dance instructor and take over the lessons. But when someone cuts in and cuts Luna&;s throat, the grieving gangster makes Peters an offer he can&;t refuse: Find the killer&;or go from having two left feet to one foot in the grave.   Now, instead of punishing the parquet, the silver screen&;s most famous song-and-dance man is pounding the pavement with his new partner&;a rumpled, middle-aged gumshoe who just wants to live to shuffle through another day . . .