I read most of these books when I was a pre-teen/teen. So, I don't remember much of the details. I do remember I like the Hercule Pirot books better than the Miss Marple books.
That said, if you like Agathat Christie there was a Dr. Who episode with "her" in it. It was pretty interesting... it attempted to explain why she disappeared for a short time and turned up at an institution.
(Sorry I don't know the name of the episode or if you can watch this show online.
Maybe this web site will help.)
BOb