THE CHARLATAN by William Hamilton (Simon & Schuster: $15.95). There's always at least one of them moving around the social fringes--the not-quite-accepted cad whose credentials for being there come through marriage, not breeding or talent. The type (you know?) who isn't above a little cheating at cards, or (when the waiter's back is turned) slipping his luncheon partner's tip into his own pocket. Poor Edgar Barnes, however, has one great attribute: patience. For 20 long years he's suffered along as the husband of fabulously wealthy Betty Bishop Barnes--the well-groomed, tag-along, kept-boy of the slightly addled Betty. His one source of comfort: the knowledge that Betty's increasing drug and alcohol dependency is steadily, insidiously, taking her to the grave. Until alas, the thoroughbred, Shanghai Pagoda, comes into Betty's life, and as his victories at the track become more impressive--the Kentucky Derby, the Preakness--his owner's health is miraculously restored. Not only is Betty suddenly threatening to outlive him by a comfortable margin but, to Edgar's horror, she's begun to see him for the expendable deadwood that he is. Does bumbling Edgar have the wherewithal to take matters into his own hands and reverse Betty's recovery? This satirical, witty novel of evil intentions has much of author William Hamilton's biting cartoon style--those slashing, black, social commentaries that he does so well for The New Yorker. Too bad that the characters in "The Charlatan," without exception, are such rotters. We need empathy with someone .