I nominate
The Mysterious Affair At Styles by Agatha Christie. It is Christie's first published novel, and hence the first with Hercule Poirot.
Quote:
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The Times Literary Supplement of February 3, 1921, gave the book an extremely enthusiastic, if short, review which stated that "The only fault this story has is that it is almost too ingenious". It went on to describe the basic set-up of the plot and concluded: "It is said to be the author's first book, and the result of a bet about the possibility of writing a detective story in which the reader would not be able to spot the criminal. Every reader must admit that the bet was won."
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Quote from Wikipedia.org
It is also widely available free, or at very reasonable prices in a veritable rainbow of formats.