The Red Triangle: Being Some Further Chronicles of Martin Hewitt, Investigator, by Arthur Morrison (Paperback)

The Red Triangle collects the final 6 adventures of detective Martin Hewitt:

THE AFFAIR OF SAMUEL'S DIAMONDS
THE CASE OF MR. JACOB MASON
THE CASE OF THE LEVER KEY
THE CASE OF THE BURNT BARN
THE CASE OF THE ADMIRALTY CODE
THE ADVENTURE OF CHANNEL MARSH