Given the woeful sales for “serious fiction,” the average literary novelist would probably be delighted to receive the critical thrashing that was recently unleashed upon Martin Amis and his new novel, “The Pregnant Widow” (Knopf, 384 pp.), by Michiko Kakutani in the New York Times — if only as proof of an otherwise obscure existence. But neither by birth (he is the son of Kingsley Amis) nor by talent can Mr. Amis possibly be construed as an average novelist.