The Opera ghost really existed. He was not, as was long believed, a creature of the imagination of the artists, the superstition of the managers, or a product of the absurd and impressionable brains of the young ladies of the ballet, their mothers, th box-keepers, the cloak-room attendants or the concierge. Yes, he existed in flesh and blood, although he assumed the complete appearance of a real phantom; that is to say, of a spectral shade.
Summary: The Phantom of the Opera sits squarely in the Mystery genre of stories, but it has a number of twists that together combine to make it unique.
(1) It is set up as a historical mystery. By this I mean more than a mystery taking place in a past time. The narrator is laying out his solution to a mystery that occurred some thirty years before. Thus it requires piecing together in the way a historian pieces together a historical narrative. While there are other historical mystery works -- Josephine Tey's The Daughter of Time is probably the most famous, although it's one I haven't read -- this is not a common route to take in mystery writing, I imagine because it is difficult to maintain immediacy.
(2) While there are several murders in the course of the story, none of them is the primary crime to be unraveled. That is the disappearance of Christine Daaé. And even that is a secondary issue; none of the crimes is the central mystery of the work.
(3) The story continually plays up the fantastic and supernatural in its descriptions -- and then undoes them by uncovering entirely natural and often mechanistic explanations of them. While this in itself is not at all uncommon, I don't think there's any other major mystery tale that does it on anything like the scale on which this book does it. There are layers and layers and layers of it. The great danger of this kind of approach is Scooby-Doo-ism, in which the story builds up a heavily fantastic appearance whose discovered explanation is a banal and wholly inadequate non-fantastic explanation, a just-so story that is little more than glib handwaving. Phantom avoids this, I think, by heavily drawing on aspects of the world that we take to be natural but nonetheless still highly suggestive: trapdoors, strange torture machines, underground lakes, secret passages behind mirrors, Orientalism, and the like. And, more important than this, it links them up in such a way that the natural solution often gets you immediately into something even more fantastic-seeming, so you don't really have time to question the adequacy of the explanation, and even if you did, the fact that the natural solutions so often bring more fantastic elements with them underlines the fact that they aren't the complete explanation.
There are occasionally stories that do approach this level of complexity in the interplay between fantastic and natural: the novels of Tim Powers. But Powers's movement is in entirely the other direction, because rather than giving us the apparently fantastic and showing us an elaborate natural explanation, he shows us the apparently natural and gives us an elaborately fantastic explanation. But the level of complexity being similar, I think one could very well say that Phantom's closest cousin is something like Declare.
(4) The story works very hard to paint the Opera Ghost both as a monster and as someone to be pitied, and merges the two surprisingly well.
Leroux's The Mystery of the Yellow Room spawned an entire subgenre of imitators, the locked-room mysteries. But The Phantom of the Opera, while far more famous, has no such imitators, and part of this, I think, is that it is inimitable -- there are so many unique features that are so essential to the story that if you pick them apart you get an entirely different kind of story. The combination of this with lush description and careful characterization certainly puts The Phantom of the Opera in the top tier of the greatest mystery stories ever written.
And turning to his audience M. Mifroid delivered a little lecture on police methods.
"I don't know for a moment whether M. le Comte de Chagny has really carried Christine Daaé off or not... but I want to know and I believe that, at this moment, no one is more anxious to inform us than his brother....And now he is flying in pursuit of him. He is my chief auxiliary! This, gentlemen, is the art of the police, which is believed to be so complicated and which, nevertheless, appears so simple as soon as you see that it consists in getting your work done by people who have nothing to do with the police."
Recommendation: Highly Recommended.