I have only read one other "The Mystery of the Yellow Room" and noticed some I guess 'similarities' between it and Phantom, I won't spoil it if anyone is interested in reading the book.
synopsis:
The main character's name is Joseph Rouletabille, an 18 yr. old reporter/ detective (he's not actually a detective he's just really smart) and his friend the narrator (Leroux) the framing of the story is told in the same fashion as Phantom, though I image all Leroux books are. They investigate the assault of a woman in the mysterious yellow room, that is locked from the inside with no secret entrances (no pivoting mirrors this time) and a man that stalks her, and literally kills to get close to her. Sound familiar?

Joseph Rouletabille works for the same paper (L'Epoque) that Erik's death was published in ("Erik is Dead")
It also takes place in Paris, though in 1892. (so about ten years after Phantom)
There is a stalker/ want to lover/ murder... character
A boyfriend character (Robert Darsac)
Most of the drama takes place in one area (Chateau du Glandier, or the DonJon)

Since this book was written before phantom I think the similarities are incidental, or are a throw back to this book. Either way the were fun to notice!

Are there any actual BOOKS, or are they just on Kindle, because I collect classic novels, and i want to OWN the book.

Yeah there is, some can be pretty pricey though, you have to remember most of his novels are out of print or are specially printed by small book companies and it won't be cheap. Only Phantom is ubiquitous. But I got a first addition of "The Perfume of the Lady in Black" which is the sequel to "Yellow Room" (mention in my first comment) for a reasonable price. Though sadly the rest of the series has not been translated. (If you can speak French this is not an issue)

Since I have now read more of Leroux's novels I will correct myself in saying that his framing style is NOT always like Phantom; Yellow Room and Lady in Black just happen to be. In "Lady in Black" Leroux inserts himself as a character (actually calling himself by name) to a very young Rouletabille at about the same time Phantom would have been taking place, though he never actually mentions the Opera house by name he says to him "you will never know the opera boy" and takes him under his wing. At that time "Leroux" was working for L'Epoque, the paper Rouletabille would later work as well as publish Erik's death in.
- relevance no, phangirl squeal hell ya!