I read this in January but am reviewing in August, so this should be fun.
A family is murdered in their home by their illiterate housekeeper, after one of the family members, Melinda (?) discovers her dark secret. Melinda is generally a good egg and becomes curious about their seemingly perfect-in-a-robotic-slash-sociopathic-way housekeeper, Eunice, with that awkward, condescending, of-the-people attitude that rich people adopt once they read about socialism and stuff. In Melinda's misguided attempt to learn more about Eunice, Melinda learns Eunice's monstrous secret - she cannot read! Eunice is sensitive about this issue, which is understandable, I suppose, but instead of doing something normal like quitting her job in a haze of embarrassment, or just fessing up and asking Melinda to help her learn to read, she kills everyone in the house instead. There was some other random woman involved who helped her murder everyone but I can't remember the details - did she turn on her and kill her, too?
Ruth Rendell is well-regarded as one of the best English mystery novelists, but this is the third book I've read by her and I find her very blah. Perhaps I am not deep / smart enough to understand all the psychological underpinnings and the examinations into the human psyche or whatever, but when reading her, I find myself sighing and wishing people would just Get Over It. This may mean I am failing as a human being, but I had no sympathy for Eunice and her predicament. However, I did learn an important lesson: Never befriend poor people because they will just murder you and your entire family.