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"): Harry Potter and the Deathly Hallows is the seventh and final novel of the "Harry Potter" series, written by British author J. K. Rowling.
Relevant evidence: Yes