What is the author's style in The Phantom of the Opera by Gaston Leroux?

The Phantom of the Opera is told from the point of view of a narrator whose name is never given, who is examining the events of the novel thirty years after the fact. The Preface gives details of his search: how he examined the records of the opera library, interviewed people who had been present at the time of the story (including Little Meg Giry and the Persian, whose name is withheld but who proves to be a major part of the action in the book's final chapters), and examined a skeleton found in the catacombs under the opera house, assuming it to be the remains of the phantom. Throughout the course of the novel, this narrator sometimes makes his presence felt, with statements like "I assume" and "we know now that," but for the most part he stays out of the story and relates the facts as a third person narrator would.