Hi. There's a huge amount of fanfiction on this site:
To try to get it down to what you are looking for:
Click "Phantom of the Opera."
At this point, you still have about 11,000 on offer! So next, click the "Filters" button above the start of the list. Where it says "world," you can select "book" - eliminating stories based on the movie or musical. While in filters, you probably want to select the language as English (it's an international site) and the status as "complete," so you don't fall into an unfinished serial. You can also select type of story, main character(s), rating etc.
How well this works depends on how accurately the authors listed their stories. Also, you have no idea of the quality, which varies wildly. Some stories have reviews, which may help a little, but be warned that some reviews contain spoilers. (Personal plug: I have a few stories on there, in the name MarySkater, but most of mine tend towards the musical rather than the book.)