What to read after Poirot's Early Cases?

With his career still in its formative years, we learn many things about how Poirot came to exercise those famous 'grey cells' so well. Fourteen of the eighteen stories collected herein are narrated by Captain Arthur Hastings—including what would appear to be the earliest Poirot short story, 'The Affair at the Victory Ball,' which follows soon on the events of The Mysterious Affair at Styles.Two of the stories are narrated by Poirot himself, to Hastings. One, 'The Chocolate Box,' concerns Poirot's early days on the Belgian police force, and the case that was his greatest failure: 'My grey cells, they functioned not at all,' Poirot admits. But otherwise, in this most fascinating collection, they function brilliantly, Poirot's grey cells, challenging the reader to keep pace at every twist and turn.