I realize that the main campaign setting book/boxed set is pretty important to get for any campaign setting. Besides that, however, what has the most information about running a Planescape game? What is absolutely essential, and why? I realize that the game is being updated here but I have always enjoyed the 2e materials I've encountered. I'd like to get the most important bits. I'd appreciate any input from gamers experienced with Planescape; I'm fascinated by the setting but don't know all about it yet.
Thanks in advance.
Kismet
Short answer is: there is no essential material.
Long answer:
You're perfectly well off using material from the internet if you decide to run a post-Faction War game, so to speak. If you are creative, you can fill in the blanks yourself, and use stuff from Planewalker and the Mimir to get the rest.
However, there are certain books that are both great background reading, and perfect for "getting the setting". You might not use them directly (although there are wonderful modules to use, if you haven't already) but they can change your style of DMing in a wonderful way if you read them.
I'll rate the books in the order that is most important to me (after the campaign setting):
Uncaged: Faces of Sigil - the best written book of Planescape...incredible characters and intrigue you can't shake a mephit at
Planes of Conflict: Boxed set. Describes the "neutral" planes, good and evil. Sets the mood for the other boxed sets, even though it wasn't the first one.
The Factol's Manifesto - the description of the 15 standard factions, which sets the theme for all sects and factions after that
Planes of Xaos - the first of the boxed sets and a wonderful read. There's more information there than I have been able to use in 8 years of Planescape. Like the other boxed sets, it's by no means essential, but it makes running a believable game in those planes all that easier.
Planes of Law - the last of the triad, and funnily enough the easiest to use. Has booklets for each individual plane. All I said about the other two goes for this one as well.
The Planewalker's Handbook - you actually don't need the campaign setting that much if you get this book...it has a lot of essentials, and all of the style
My favorite modules:
Harbringer House - the quintessential Sigil module, in my mind
Dead Gods - Still my favorite Monte Cook adventure (almost tied with Hellbound, below). You'll never see divinity in the same way. This module will make you want to get the Guide to the Astral Plane, and you won't regret that either.
Hellbound: the Blood War. A lot of people tend to think of fighting fiends when they think of the planes. Hellbound makes them rethink that. An epic campaign for Planescape if there ever was one.
Doors to the Unknown - four adventures that tie-in to a mini-campaign
Well of Worlds - a lovely collection of adventures for pretty much any occasion.
"Guru" books (by no means necessary, but they deepen your understanding and pleasure in the setting):
Fiends: Faces of Evil - you'll never look at the lower planes in the same way. My second favorit planescape book.
In the Cage: A guide to Sigil - a lot of Sigil tidbits that you might otherwise miss. Only for lovers of the Cage (such as myself).
On Hallow Ground - I actually only bought this a week ago and I'm loving it. It just goes to show that you don't really need all the books to understand the setting.
My, that was a confusing answer, wasn't it? There's more planescape stuff than this, obviously, but I pretty much described my current favorites right there, keeping in mind that you are pretty new to this stuff. I still stick by my original answer, though. You don't need any of it, even if it's all good (the Planescape label 99% guarantees a wonderful product) - the free, fan-made stuff is actually amongst the best and the most imaginative.
Just browsing Planewalker, the Mimir, On the Wings of Mephits and others (like Kriegstanz and Dream of Dreams), you can get enough material to create a perfect Planescape game. At this point the setting has really outgrown the original books.