The only good thing about The AC Companion by Sanders and Lovallo was the lists in the back. Other than that it's not worth picking up. It had so many inconsistencies it wasn't funny. Much of the biographical information about AC was botched as well.
I very rarely say this about a book, but reading the AC Companion was like trying to pull teeth from a hamster.
"The Agatha Christie Mystery" by Derrick Murdoch on the other hand was a very excellent and prolific piece of writing.
It was written just before "Autobiography" was printed, so Murdoch used what references that were on hand at the time. Most of what he used as references were articles from newspapers which are hard to find today, and out-of-print books dealing with detective fiction.
One of the high points of "Mystery" is that it reprints Pamela Stewart's "Elegy For Agatha Christie" from New Yorker Magazine. Another bonus of this book is a portrait of AC by Lord Snowdon.
It also contains the 1976 version of the lists, i.e. plays, books, films.
I would consider "The Agatha Christie Mystery" by Derrick Murdoch to be an essential read for any Christie fan.
Keep on reading.