The Angela Marchmont Mysteries are standalone mysteries that can be read in any order up to and including Book 8, The Trouble at Wakeley Court, although since there is a low-key story thread running through them, in order to get the best out of them you might prefer to start at Book 1 and work through them numerically.
The Freddy Pilkington-Soames Adventures can also be read in any order, although again there is a bit of a story thread in the background (particularly in Books 4 and 7, A Case of Duplicity in Dorset and A Case of Robbery on the Riviera), so you may prefer to read them numerically.
Note that if you’re coming to Freddy first and would like to go on to read the Angela books afterwards, there are major Angela spoilers in A Case of Robbery on the Riviera, in which Angela makes a cameo appearance as Freddy’s sidekick.
The books written so far all take place between Oct 1925 and Oct-Nov 1930. The novels from both series are in chronological order by date of release, but I skipped back and forth in time when writing the Angela short stories.
However, if you’re a purist and would like to read them in the order they happened, you can refer to the list below.
Date set Title
Oct 1925 The Man on the Train (short story)
Oct 1926 The Murder at Sissingham Hall
March 1927 A Pinch of Strychnine (short story)
May 1927 The Mystery at Underwood House
July 1927 The Treasure at Poldarrow Point
Sept-Nov 1927 The Riddle at Gipsy’s Mile
Dec 1927-Jan 1928 The Incident at Fives Castle
Feb 1928 A Question of Hats (short story)
May 1928 The Imbroglio at the Villa Pozzi
June 1928 The Problem at Two Tithes
Oct 1928 The Trouble at Wakeley Court
Nov 1928-Jan 1929 The Scandal at 23 Mount Street
May-July 1929 The Shadow at Greystone Chase
Christmas 1929 Angela’s Christmas Adventure (short story)
Oct 1929 A Case of Blackmail in Belgravia
Nov 1929 A Case of Murder in Mayfair
Jan-Feb 1930 A Case of Conspiracy in Clerkenwell
April 1930 A Case of Duplicity in Dorset
June-Aug 1930 A Case of Suicide in St. James’s
Sept 1930 A Case of Robbery on the Riviera
Oct-Nov 1930 A Case of Perplexity in Piccadilly