Trent's Last Case by E.C. Bentley
Trent is asked to give a corespondence regarding a much publized murder case for the paper he writes for. When a close personal friend asks him to do a favor by assising the local police in this matter he decides to see for himself what is so special about this case. He discovers a mass of confusing clues that do not even lead to the correct conclusion as this clever plot, contains not one but two solutions to the crime. This novel is also known as being the start of the modern mystery novel. Dorothy L. Sayers writes the introducation.

Wow, was this book interesting. Well writen and several red hearings. I never did figure it out plus with the dual solutions I never would have.