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 novel chronicles the events directly following "Harry Potter and the Half-Blood Prince" (2005), and the final confrontation between the wizards Harry Potter and Lord Voldemort, as well as revealing the previously concealed back story of several main characters.
Relevant evidence: No