The Boomerang Clue or Why Didn't They Ask Evans by Agatha Christie

This mystery features none of Christie's usual detectives, the two people who end up detecting don't use the police, or have conventional techniques. There's a bit of a cover-up over the Evans of the title, and a double- and a triple-cross in terms of how you feel about the characters, but overall I found this mystery quite enjoyable. I particularly loved the moments when these amateurs thought they were being quite subtle and the other characters saw through their work.