Devil May Care by Elizabeth Peters

Ellie is young, rich, engaged and in love. These are the carefree days before marriage and new responsibility, and anything goes -- including house-sitting at eccentric Aunt Kate's palatial estate in Burton, Virginia. Ellie feels right at home here with the nearly invisible housekeepers and the plethora of pets, but she soon realizes that there are disturbing secrets about the local aristocracy buried in a dusty old book she has carried into the mansion. And her sudden interest in the past is attracting a slew of unwelcome guests -- some of them living and some, perhaps not. And the terrible vegeance that Ellie and her friends seem to have aroused -- now aimed at them -- surely cannot be...satanic.