Question: Who wrote an unofficial sequel-series of J.K. Rowling's Harry Potter universe, that enters around the character of James Sirius Potter who makes a fleeting appearance in the  final novel  of the "Harry Potter" series, written by British author J. K. Rowling?
Sentence (from "Harry Potter and the Deathly Hallows"): The title of the book refers to three mythical objects featured in the story, collectively known as the "Deathly Hallows"—an unbeatable wand, a stone to bring the dead to life, and a cloak of invisibility.
Relevant evidence: No