Originally Posted by DrNefario
The Murder of Roger Ackroyd is the third Poirot. It probably isn't really necessary to read them in order, but there are only two to read beforehand.
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.