Upon reading the novel, I did not expect to recognize the story from what I knew of the stage production and was pleasantly surprised to find that much of the text had been preserved in the adaptation. From the major characters to the minor ones, even Madame Giry and her daughter Meg (whose singing parts I love since I can pull off a decent mezzo and can't get anywhere near Christine's high notes) appear in both book and score. There were also plenty of details, such as the lasso the "ghost" uses to strangle his targets and the lake beneath the opera house, that the adaptation did right. The difference that most surprised me though was how human the character of "The Phantom" is in the original novel. In the musical, he seems much less mortal - more the mysterious Angel of Music than a real man. Yet, in the book when he was given both a name, Erik, and a backstory, I found him less likable than on stage. Perhaps clouding his humanity in a ghostly mystique made his atrocious acts more acceptable, but I found his character in the book violent, frightening, and much less sympathetic. I realized that Leroux intended for Erik's voice to be his main redeeming factor - the aspect that draws Christine to him - and this obviously came through much better in the musical than in the book.
Another thing that I found really interesting in the book is the description of the organ. The title song in the musical is most recognizable by powerful and dynamic chords of organ music (DAAA-da-da-da-da-DAAA....). And though Erik is described at multiple points as playing a piano, this passage struck me as almost funny, in light of the music that ran through my head as I read:
"Contrary to what one might think, especially in connection with an opera-house, the 'organ' is not a musical instrument. At that time, electricity was employed only for a very few scenic effects and for the bells. The immense building and the stage itself were still lit by gas; hydrogen was used to regulate and modify the lighting of a scene; and this was done by means of a special apparatus which, because of the multiplicity of its pipes, was known as the 'organ'"
~Gaston Leroux, The Phantom of the Opera p.223
So although the musical got many things right, I'm not sure that the inclusion of organ music was as accurate as intended. I think this quote is also demonstrative of Leroux's almost documentary style of storytelling. At first I found it a little off-putting, but once the story began to unfold, I appreciated his commentary-type notes as both informative and educational throughout the text. Overall, I'm really glad to have read this book. As much as I love the musical, I think familiarity with the original source material gives me an even deeper appreciation of the art created in the adaptation.