Complex sentence: All of the stories in the collection had previously been published in magazines (see "First publication of stories" below) and feature her detectives Tommy and Tuppence Beresford, first introduced in "The Secret Adversary" (1922).
Split into simpler sentences: All of the stories in the collection had previously been published in magazines (see "First publication of stories" below). The book features her detectives Tommy and Tuppence Beresford, first introduced in "The Secret Adversary" (1922).