NIGHTSHADE
by DEREK MARLOWE 192 pages. Viking. $7.95.
At last, a modern novel more or less about abstinence. Nightshade's protagonist, Edward Lytton, is 40, devoutly Catholic and astonishingly, since he is four years married, a virgin. He does not mind his asexual life; indeed, he is so Victorian that he can barely imagine any other. Alas, his unfulfilled young wife Amy is not resigned to her condition. She indulges in "fantasies of liquid the color of magenta, a pomegranate redness, viscous to the touch, so that one has to lick it dry." Poor Edward...