I stopped reading this book after about a hundred pages - not because it wasn't good, it was, but because I haven't read all of Christie's canon yet, and this books is 100% geared towards those that have (or have at least read a majority of it). The author states from the beginning that there are massive plot spoilers throughout every chapter; he even goes so far as to list the books a particular chapter is going to spoil.
I didn't care about the plot spoilers; when it comes time for me to catch up on Christie's canon, I will have long since forgotten who did what to whom (I've read some of her books twice - The Body in the Library for one - and I still can't remember who the murderer was). The fact of the matter, and why I DNF'd the rest of the book, was that Christie's notebooks and the author's commentary on them are largely meaningless to anyone who isn't familiar with the particular work in question.
I did read the two short Poirot stories at the end, (I'll review them under separate posts) and what I did read at the beginning was interesting and well written. If you're a huge Christie fan and know her work well, I'd recommend this book as a way of delving deeper into how this master of mystery's mind worked. Otherwise, most of the book reads a bit like someone's coded grocery list.