When I first watched "The Phantom of the Opera" musical, I knew I was "past the point of no return" (pun intended). I fell in love with the story of Christine and her complicated relationship with her Angel of Music/Phantom of the Opera. I knew I had to read the book.
That said, a word of warning to everyone who choses to do it in the same order as I did: dismiss everything you know. Forget about the entire plot you saw in the musical, and read the book as if you know nothing. Don't expect anything. Don't compare the musical to the book. You'll get lost and confused.
While I have an extreme love for Andrew Lloyd Webber's musical, he does stray from the plot quite a bit. In the beginning, as I read, I tried to think about the movie and figure out what was going to happen next. I only succeeded in confusing myself. The plot in the musical is in a different order than the book, and, as "movie versions" usually do, it changes and/or leaves out many of the details. That said, take my advice-forget the musical, and read the book with a completely open mind, forgetting everything you think you know.
The novel is an incredible piece of literature though, and I would absolutely suggest that all fans of the Phantom read the book. It opens your eyes to the way Erik thinks and feels. You think of him as a monster, but cannot help but pity him. Your heart will break with his as you see his love for Christine, but realize it can never be. You see through the eyes of Christine, who cannot see that her Angel of Music is not what he seems until it is too late. Raoul shows am example of true love with his everlasting feelings and forgiveness towards Christine, and his willingness to do anything for her. The book will make you smile over innocent romance, gasp over magical and mysterious events, and cry over broken hearts. Gaston Leroux has written a real piece of art that was a true pleasure to read.