Erm... Technically, two. Le Phantom De L'Opera by Gaston Leroux which is the original and Phantom by Susan Kay.

I have both the French edition and the English edition of Le Phantom, but since it is technically the same book, two.

Original novel, Phantom by Susan Kay (the Bible!), and Phantom of Manhattan (which I enjoyed after I got over all the blasphemy to the original novel that Forsyth decided was the truth... Idiot...).

Also, lots of phanphiction!

The original and Phantom of Manhattan .. I'm in the process of getting the Susan Kay book I've heard nothing but good things about it.

Unfortenately just 1, but I love to read fanfictions about it. :-)

Man I've read the most 6! and get this I've never read (unpublished phan phiction) is that not bazaar? lol here's what i've read.
Phantom of the Opera by Gaston Leroux- The original nuff said XD
Phantom by Susan Kay- I think everyone knows how awesome this book is <3

Masquerade by Colette Gale- Erotica, feeds your shipping needs we all have them...

Maskerade by Terry Pratchett- this one is a parody, I liked it; Christine is REALLY dumb in it though.

Journey of the Mask by Nancy Hill Pettengill - I don't like the "and after the opera house" E/C shipping, plus they went to Louisiana and lets just say there's voodoo involved, took me a while to finish that one.

The Phoenix of the Opera by Sadie Montgomery - I didn't read the rest of the series because its a Erik/Meg shipping and I 'm not into that.