Main Character: Scotland Yard's Superintendent Battle. To be accurate, Superintendent Battle is not so much a main character as he is a unifying element in this series of books. He also appears in the Hercule Poirot mystery, Cards on the Table.
Bold type indicates a book I've read.
The Secret of Chimneys (1925)
The Seven Dials Mystery (1929)
Murder is Easy (1939), aka: Easy to Kill
Towards Zero (1944), aka: Come and Be Hanged