Of note: A Caribbean Mystery introduces the wealthy (and difficult) Mr Jason Rafiel, who will call upon Miss Marple for help in Nemesis (1971) -- after his death.
Agatha Christie is the most widely published author of all time, outsold only by the Bible and Shakespeare. Her books have sold more than a billion copies in English and another billion in a hundred foreign languages. She died in 1976, after a prolific career spanning six decades.