This book is for serious fans of Christie - it spoils many of the solutions to the mysteries and is both a deep dive and a breezy overview.
Christie left behind some 80 notebooks full of plot ideas, clue placements, puzzles, character names/descriptions, and solutions. The most interesting thing I've learned so far is that she frequently dithered between multiple solutions/murderers in books, and an idea/clue that wasn't used in one of her mysteries would crop up later in another mystery.
In fact, the entire Boynton (although she referred to them as P, planning, apparently to name them something that began with the letter P) family was first assigned to Death on the Nile, but didn't make it into the final book. Christie then built an entire mystery around the family two books later in Appointment with Death. In addition, everyone should know that the intensely annoying Salome Otterbourne's original name was "Mrs. Pooper."