Originally Posted by HarryT
The fourth, to be strictly accurate:
1. The Mysterious Affair at Styles (1920)
2. The Murder on the Links (1923)
3. Poirot Investigates (1924)
4. The Murder of Roger Ackroyd (1926)
"Poirot Investigates" is a short story collection, so it is the third novel.
I would suggest reading "The Mysterious Affair at Styles"; it's an excellent introduction to Poirot.
In that case, I will read the previous three. I just downloaded The Mysterious Affair at Styles
, I just discovered that it's free on Project Gutenberg along with Secret Adversary