There's a very interesting book somewhere inside Cornelia Read's debut, A Field of Darkness, about Madeleine Dare, a young former debutante who goes slumming in 1980s upstate New York as a reporter for a weekly paper. She comes from old WASP money and her best friend is Ellis, ''a Cheever girl, a Salinger girl.'' Read, herself a former deb, is quite perceptive in describing money gone to seed, writing of the Dare family: ''it's like looking at a herd of failed Kennedys.'' Oh, but then she wraps in a mystery plot about a decades-long unsolved murder that pops up again. It's hard to maintain interest in something so standard when the author keeps drifting toward much more compelling matters.