Walter Mosley, bestselling mystery writer and creator of gritty gumshoe Easy Rawlins, has written a dirty, dirty book. ''I've been joking around with people, saying, 'I'd like you to read my sex book,''' declares Mosley about Killing Johnny Fry (out in January). Only he's not kidding. This erotic potboiler is teeming with more hardcore scenes sodomy, orgies, and even a nod to incest than you could shake a sex toy at. (We stopped counting at about a dozen.) And while the author prefers to call his noir crime story ''a sexistential novel,'' there's no denying that the prurient details might turn off as many fans as they will most assuredly turn on. ''I know that sex throws people off,'' admits Mosley. He's quick to shoot down rumors that he signed up with Bloomsbury books only after longtime publisher Little, Brown blushed at the content, but adds, ''It would be nice if someone banned it.... That would be helpful.''