All right, so there are a pile of PotO books out there, some suck, some don't. This thread is here to offer up reviews from other phans about which books are worth reading and which you should just leave alone for your own sanity's sake.
Le Fantome de l'Opera, by Gaston Leroux. THE ORIGINAL. If you haven't read this yet, go to it. Now....I'm serious. I'll wait. JustHuddy was good enough to post a link so that you can read it online FOR FREE. Ya really got no excuse with that there. So get thee hence and make with the reading dammit!
Phantom, by Susan Kay. This one generally ranks a very close #2 to the original Leroux. This, very basically, is a biography of Erik's life, sometimes from his point of view, sometimes from others. You can also see some ALW influence meandering around in there. Definitely worth reading.
Angel of the Opera, by Sam Siciliano. One of the Sherlock Homles meets the Phantom ones. This is the good one. If you can find a copy of this, absolutely pick it up! It's a fun read, though the final ending might squick a few phans. Overall it's pretty good, though.
Phantom of the Opera, by David Bischoff. This version is....interesting. I'm pretty sure Erik was smoking crack in this one. If you find it cheap in a bargain bin you can read it for gits and shiggles, but I wouldn't wander too far out of your way to read this one.
Phantom of Manhatten, by Frederick Forsyth. No! Just, no! This is what the ALW sequel was originally supposed to be based on. If I weren't so heavily against the burning of books, this particular novel would be charcoal. Really, it's that bad. Your soul will bleed. And this goes for if you're an Erik OR Raoul shipper. Takes talent to piss off both camps at once, believe me.
The Canary Trainer, by Nicholas Meyer. This is the other Phantom meets Holmes I've read. This is the kinda sucky one. Holmes is a drug addict (ok, so he actually was a little bit, but this is bad) and Erik apparently can't talk without his mask and is Really Annoying. For the love of god go for the Siciliano instead.
Phantom of Paris, by Gwenith M. Vehlow. This is glorified phan phic. I was not very impressed with it. Secondary characters would appear for one or two pages, then vanish, never to be seen again. Erik acted unrealistically, and ended up finding Jesus, which was all kinds of creepy. But that may just be the pagan in me talking, though he lost almost all of his bite. I'd give this one a miss, personally.
Journey of the Mask, by Nancy Pettengill. Yet another sequel novel. This one is all right. Basically Erik snatches Christine and hauls her off to America. New Orleans to be specific. He gets involved in the hoodoo down there, and Christine does eventually fall in love with him, mostly because he's the only familiar face in a strange new city. It's not a great piece of work, but I've definitely read worse. If you make it through the Leroux, Kay, you can put this on your list for when you need a phix.
Progeny, by Becky Meadows. Probably the best sequel you're going to find. This book follows Christian, Christine's son as he discovers the secrets about his and his family's past. It's pretty well written and actually explains where it's coming from, which can be rare in sequels. I'd say this is one that you should go ahead and read.
Music of the Night, by Suzy McKee Charnas. This is technically a short story, and it will completely piss off any and all angry feminists. The thing I remember most about this version is that there was some smut, and Erik's kind of a stubborn son of a bitch. It's ok, at least it's short. Another one that I'd only suggest grabbing if you find it in the bargain bin.
Unmasqued, Colette Gale. Uhm...yeah. This is erotica. No kiddies under the age of 18 should even THINK about buying this book. It's really kind of pathetic. It wants SO badly to have a plot of it's own it can almost taste it, but it just doesn't quite work. Another thing that annoyed me about this was that it was obviously heavily influenced by the 2004 movie. Or maybe that's for the best considering what it ended up being. Just as a warning, this book might end up ruining harp music for you for quite some time.
So there you have it. These are all the Phantom novels I've read that I can think of at the moment. Bear in mind that some of these I have not read for damn near 5-10 years. Everyone, please add to this thread. If you've read one of these yourself and have a different opinion, let everyone know. I know my tastes probably do not run the same as several of the phans on here and stuff that I wasn't that fond of someone else might absolutely adore. Hope this helps when deciding what piece of Phantomy goodness you should look into next! :)