Josephine Tey: Inspector Alan Grant Mysteries

Josephine Tey is the pseudonym of Elizabeth MacKintosh, who also published under the name of Gordon Daviot.
Main Character: Inspector Alan Grant of Scotland Yard
Setting: England and Scotland
Bold type indicates a book I've read.

The Man in the Queue (1929), writing as Gordon Daviot
A Shilling for Candles (1936)
The Franchise Affair (1948)
To Love and Be Wise (1950)
The Daughter of Time (1951)
The Singing Sands (1952)

Agatha Christie: The Superintendent Battle Mysteries

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.
Setting: England
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