Once the sexy bad boy of English letters, Amis, at 54, seems more and more like its dirty old man. In the first chapter of ''Dog,'' his first novel in six years, ''dream husband'' Xan Meo is attacked by henchmen of mysterious thug Joseph Andrews, triggering a personality change that has him mounting his exhausted wife night and day and showing unseemly interest in his 4-year-old daughter. Andrews may also have something to do with the troubles facing King Henry IX of England, who receives a nude photo of his teenage daughter. Who took it? What do they want? Haphazardly interspersed with the tribulations of these family men are the antics of Clint Smoker, a stock Amis antihero, this time a genitally under-endowed tabloid hack. A few choice bits of writing adorn this rambling mess, but you have to wade through acres of muck to get to them.