Murder at the Mansion — Bob Moats

In this third of the Nick Cain series, Nick and his lady invite Detective Bill Edwards and his wife to the mansion for a fancy dinner. Things start to fall apart after dinner when a body is found on the property, Local police are called but nothing is found pointing to a killer. Later after everyone is getting ready for bed another body is found on the property and it's very similar to the fiirst. The reason for the murders points to another reason, one that can harm Nick's girlfriend Sally. This book is a novella, shorter than a novel but longer than a short story.