A Plague on Both Your Houses / An Unholy Alliance

There are two novels in one omnibus. Susanna Gregory writes historical, detective fiction about Cambridge in the 1300's. The first book chronicles the arrival of the bubonic plague to Cambridge and the horrendous death toll which follows, along with a some other murders, motivated by something entirely different. The second one deals with events that followed the plague and what people turned to following it--satanic cults instead of traditional religion.