LIVE FREE OR DIE by Ernest Hebert (Viking : $19.95 ; 388 pp . ) . Talk about mismatched lovers! Can Frederick Elman, a dreamy-eyed drifter who is forced to return to his New Hampshire hometown (to take over his ailing father's trash- collection business!) find happiness in the arms of the well-born Lilith Salmon, who is equally trapped by her family ties? "Live Free or Die" is the fifth, and final, novel that Ernest Hebert has built around the fictional town of Darby, N. H., and it bristles with bright characterizations and right-on social commentaries in the clash between rural natives and metropolitan money interests.