The Murder at Hazelmoor (The Sittaford Mystery) by Agatha Christie

I was sure I knew the solution to this mystery all the time I was reading it--and as it turns out, I was dead wrong. Christie doesn't use one of her usual detectives for this story, but instead mostly follows the work of the fiancée of the accused, Emily Trefusis. I thought this mystery was particularly well-done; I love mysteries that have all sorts of secondary and tertiary plots running through them as well.