If you can track down a copy of this novel, it is well worth it. First published in 1935, it features Dickson Carr's detective Dr Gideon Fell solving not one but two interlinked locked room mysteries.
It is set in 1930's London. In the first mystery, Professor Grimmaud is found dying in his study, the man who just entered the study has vanished, the snow on the window ledge is undisturbed and the door was watched all the time.
Later, a man is walking down a snow covered street, a shot is heard and the man falls down, on examination he is found to have been shot at close range. Yet again, there are no other footprints and there are also three witnesses who saw no one.
Whilst the story and the solution are both excellent, what was of especial appeal to me was chapter 17 in which Fell breaks the fourth wall and debates the forms of locked room mysteries and how they can be solved or created.
Fell as a detective was one part of this book which was a bit of a let down for me. He is too pantomime for my tastes, to close to Sherlock to have his own identity and, although I know he was modelled on G K Chesterton, I could not help but feel he was just a poor man's Sherlock clone.
This is the first Dickson Carr book I have read, and, unfortunately, many of his novels are out of print. However they can be sourced second hand quite cheaply. I have just purchased four more on the basis of this book alone.
For those who can't find a copy and are interested, excerpts from the locked room lecture in chapter 17 can be read here.