Gaston Leroux’s Le Fantôme de l’Opéra (The Phantom of the Opera) was first published in France in 1910, and in English in 1911, and is of course a must-read for any phan!
If you read French, obviously, read it in French! Most English-language mass-market paperback editions still use the first English translation, by Alexander Teixeira de Mattos, and this is the one Lloyd Webber, Richard Stilgoe and Charles Hart read when creating the musical – however, it leaves out a number of passages found in Leroux’s original. There are a number of modern translations of the whole novel now available, including translations by David Coward and Mireille Ribière, and Leonard Wolf. If your copy doesn’t name a translator, it’s the de Mattos translation (which is now out of copyright).
The advantage of the de Mattos translation is that Phantom fans will recognize several phrases that were used in the lyrics of the show, which adds to the poetic impact (such as “Little Lottie thought of everything and nothing”). But you’ll be missing several essential paragraphs.
My recommended edition of Gaston Leroux’s novel is the version annotated and illustrated by Michael Grant Kellermeyer (published by Oldstyle Tales Press), which gets around this problem by using the de Mattos translation, but restoring the cut passages. The annotations are clear, extensive, and will really add to your appreciation of the book. They’re also based on years of research, including research done by phans who’ve examined every aspect of the novel in depth.
The most highly recommended of modern translations is the David Coward version, from Oxford World’s Classics, as it’s unabridged and largely accurate to Leroux. Wolf is unabridged, but contains mistranslations.
Various other translations claim to be unabridged, but in fact still cut out passages, or are so inaccurate they’re more adaptations of the text than translations. The Lowell Blair translation is sadly not complete, Ribière quite inaccurate, and the Lofficier version makes so little attempt at accuracy it completely makes up lines that aren’t in Leroux at all, such as claiming Christine’s age is 17 (in fact from the internal chronology of the novel, it’s clear she’s 20 or 21). If you’ve only read the Lofficier Phantom, you haven’t really read the original novel at all. Sorry!
For a detailed breakdown of the different translations, and how accurately or otherwise they represent Leroux, we highly recommend Phantom scholar Caitlin Freeman’s blog articles.