I am summoned!
Nobilis is very easy to buy for, as it's essentially a one-book system. The most current book is 3e, and it has all the new rules and system bits that you'll need. Like most other Nobilis books, it's also hard to find a hard copy, but the pdf is easily available at Drivethru RPG.
Other books to look at- 2e is hard to find, but the pdf can be got online. It can be useful to elaborate on certain setting elements, but you have to keep in mind that there are a number of large and small changes between it and 3e.
The recently released
Minibook 1i: A Diary of Deceivers is a splat about everyone's favorite enemies of reality, and a good read if you want to know more about their metaphysics and want formal rules on playing Deceiver PCs. I'd recommend it, but I'm also obsessed, so take that as you may. It's 3e material, unlike-
The peculiar books! 2e had two peculiar books released online for free, titled
Unlikely Flowerings and
Creatures Clothed in Strangeness. The first details the arts of Alchemy and High Summoning, which are alternate attributes that powers can take, and are more or less compatible with 3e (kind of. There are mechanical issues to address, and several that don't exist in 3e, but the basic ideas and structure can be adapted fairly easily.).
Creatures Clothed in Strangeness gives more setting info and looks at Noble society, introducing fashion and the Accords of Babylon. Keep in mind that these books also use the 2e setting, so there are differences between 2nd & 3rd edition books.
1e is a little pink book that you probably won't find, and is of little interest unless you're a collector or really want to play 1e Nobilis, which is mechanically a more poorly organized version of the 2e book.
TL:DR- The core 3e book should suffice, but if you're looking for more, I'd recommend the
Minibook 1i, and the 2e core if you can find it. I'd also check out the
Elegant Discussion threads at RPG.net.