Phantom of the Operetta (1986)
by Mary Blount Christian
The Determined Detectives series is a set of mystery books for kids, all published in the eighties. They're all based in classic mystery fiction, too - in addition to borrowing from Msr. Leroux for this one, Christian's other works are drawn from the likes of Agatha Christie and Dashell Hammett.
The story revolves around two kids, Fenton P. Smith and his sidekick Gerald Grubbs, who decide to solve the mystery of the supposed ghost that is haunting the Scudder Civic Theater (with a little bit of help from Mae Donna Dockstadter, an icky girl that they totally don't like, for real). The true mystery, regarding why on earth their parents would want to force them to go through life with those names, is not addressed. The writing style is surprisingly engaging, and cute without being cutesy; it's not really at a higher reading level than the Stine book of almost a decade later, but it does give its audience more credit for intelligence when it comes to sentence structure and content.
The typical child's dislike of (or, at least, incomprehension of) opera is present, since Gerald's aunt Annie is an opera singer and the boys are deeply uninterested in the "soprano noises" she keeps making. They are also none too pleased when she presses them into service in the childrens' chorus of her upcoming performance of Gilbert & Sullivan's The Pirates of Penzance, though even they are grudgingly forced to admit later that an opera that's about pirates and fighting might not be so bad. Penzance is a great choice for a kids' book; yeah, it's not Gounod or anything, but I can't think of any operatic experiences better able to introduce kids to the art form than the Gilbert & Sullivan comedies.
The general problem is that a ghost appears to be haunting the theatre, knocking holes in the walls and generally making a nuisance of himself. He's described as wearing a long robe and hood, which is usually the hallmark of the Ye ban ge sheng versions of the story, but in this case, I think all the influence comes from Leroux's novel; the story is very much focused on trying to find out if the ghost is real or merely a person, and the mystery setting echoes Leroux's style despite the lowered reading level. Annie, perpetually forgetful, even calls Gerald "Gaston" at one point when she's forgotten his name.
The solution to the mystery is, of course, visible a mile away, and the Phantom is very obviously just a mortal man (and not one who really resembles the original Erik very much in any way), but the scene in which the kids apprehend him is laughably cute, and the entire book is pretty amusing. It's so far removed from the heavier ideas of the original book that it could probably have served as a stand-alone without any connection to the Phantom story at all, but Christian obviously borrowed the basic premise from the original, even if she pared out the love story and all of the deeper layers.