Hey there! Nice work in reading The Phantom of the Opera! I haven't read the book but one thing I will say, though, is that English has become progressively informal. If we were to look at English from 60+ years ago and English from today, you would see that the level of English has evolved, and some would even say that it is been "dumbed down." In my opinion, universities today have a very low standard. There are bills being passed today in the USA that seek to remove standard testing for reading, writing, and math. That being said, The Phantom of the Opera was written in 1909. So yes, that English may not all be used today (and I think that is sad). If you want to grow your English so that you have greater knowledge of a higher level of English then go for it! If your goal is to be able to understand and be understood in most circles today then it may not be a priority for you :)

It was originally in French, right? So a lot may depend on the translation.

if you have a C1 level, read everything. :)