This book is a complete collection of "all" 50 short stories featuring Poirot, all of which have appeared in smaller Christie short story collections. ("All" is not strictly true; more about that shortly.) Since there are so many, I won't follow my usual procedure of discussing each individual story; for that I refer you to the individual collections. Instead, I'll address what this particular omnibus has to offer.
In the table of contents, the stories are grouped according to the collection in which they first appeared:
_Poirot Investigates_ (14 stories, all 1923)
_Dead Man's Mirror_ (4)
_The Regatta Mystery_ (5 Poirot stories out of a larger collection; throughout the 1930s)
_The Labours of Hercules_ (including the Foreword)
_Three Blind Mice_ (3 Poirot stories out of a larger collection)
_The Under Dog_ (8, all but "The Submarine Plans"; most 1923, one 1926)
_Double Sin_ (4; 1923 and 1928)
In other words, this book contains the complete contents of _Poirot Investigates_, _Dead Man's Mirror_ (a.k.a. _Murder in the Mews_), _The Labours of Hercules_, and all but one from _The Under Dog and Other Stories_, and the Poirot stories from the various 'mixed' collections.
I don't care for the organization of the book, because it adds nothing not provided by the individual collections, which when assembled were split up with little regard to chronological order. For instance, the 4 stories of the _Double Sin_ collection appear last in the book, but 2 of them are 1923 stories, just like all the stories from _Poirot Investigates_, and the other 2 (1928) predate *all* _The Regatta Mystery_ stories. Two of the stories from _The Under Dog_ were written prior to "The Adventure of 'The Western Star'", which refers to those cases, but the reader is left to jump from the beginning to the end of the book if he or she wants to read the stories in order. While this is no worse than the individual collections, it's no better, either. No index, chronology, or annotation is provided.
The collection claims to have "all 50" Poirot short stories, but that isn't strictly correct. The stories that were omitted were the earlier-but-different versions of some of the stories that were included - that is, stories that were rewritten and retitled from the form in which they were originally written, but retained the same basic structure. "The Second Gong" from _The Witness for the Prosecution and Other Stories_ was omitted; it was an earlier version of "Dead Man's Mirror", although it didn't see print until after its more famous alter ego. "The Submarine Plans" from _The Under Dog_ was omitted because its alter ego, "The Incredible Theft" from _Dead Man's Mirror_, was included. "The Mystery of the Baghdad Chest" (misspelt, however) was selected over its Spanish Chest alternate. To be fair, in the latter two cases the similarities between the stories are *extremely* pronounced. On the other hand, though, both "Murder in the Mews" and "The Market Basing Mystery" were included.
Something like Baring-Gould's annotated treatment of Sherlock Holmes would be nice, someday, but this omnibus isn't it. It serves a purpose - making it easy to get "all" the Poirot stories in one container - but with a little effort could have been better.