Death Comes to Pemberley by P.D. James
All of Pemberley is getting ready for the annual Lady Anne Ball, and all seems to be going as planned. That is, until the carriage with Elizabeth's sister Lydia shows up. She's all in a tizzy, going on about gunshots in the woods and begging someone to find her husband, fearing for his life. When the search party finds Wickham, he is alive. However, he's covered in blood and standing above the body of his best friend, Captain Denny. So begins the mystery of the murder of Captain Denny, which will certainly bring scandal on Pemberley and the Darcys, and might end up with Wickham hanging from the gallows. This is "Death Comes to Pemberley" by P.D. James.