Other Sleuths of Agatha's
The first edition dust-jacket of The Secret Adversary, published in 1922 by The Bodley Head in London. This novel marks the first appearance of new heroes Tommy & Tuppence Beresford.
Hercule Poirot and Jane Marple aren't the only detectives to adorn Agatha Christie books. "Other" sleuths even appear in Poirot books but are recurring characters in their own "write". In fact, some of these recurring sleuths were created even before Miss Marple's appearance.
In the 1920's, Colonel Race, Superintendent Battle, and Tommy & Tuppence Beresford appear. Colonel Race is the secret service agent that appears in four novels spanning the years 1924-1945. Christie created Superintendent Battle to appear in five stories, from 1925-1944. This Scotland Yard man and Race appear together once and also with Hercule Poirot. We also have been given a great pair of detectives in the form of Tommy & Tuppence Beresford. What is incredible of this couple is that they make it out of the late 1940's. Their adventures consist of only four novels and one collection of short stories, spanning the years 1922-1973. No other detective (other than Hercule Poirot) covers such a length in years.
The first US edition dust-jacket of The Seven Dials Mystery, published in 1929 by Dodd, Mead and Company in New York. Superintendent Battle appears in this, his second book, a direct sequel (very rare amongst Christie's books) of the mystery novel The Secret of Chimneys.
The next batch of other detectives arrives in 1930. We get Harley Quin and his associate Mr. Satterthwaite, Parker Pyne, and the great Ariadne Oliver. The mysterious Mr. Quin and his companion appear in short stories only--first in 1930, with the last adventure in 1971. The retired government employee Parker Pyne (now a detective) appears in short stories also, in 1934 and 1939. Among Christie's fans and readers, probably the most well-known of the "lesser" heroes is Ariadne Oliver, the detective writer. This self-caricature of Agatha Christie first appeared in 1934 (with Parker Pyne) and last appeared in 1972 (with Poirot, totaling 6 books with him).
The first edition dust-jacket of The Pale Horse, published in 1961 by Collins. This is the only Ariadne Oliver novel without Poirot.
So, here you'll be able to learn the stories and methods of these other sleuths and heroes of Christie's fictional universe. Incidentally, some of the best stories of Christie's (for me) are ones with these characters. Suggested reading would be Battle's The Seven Dials Mystery, Race's The Man in the Brown Suit, Oliver's The Pale Horse, and the Beresford's The Secret Adversary.