That's book 2, you can actually just play it from the start of the game if you just click on the fire nation on the map during the intro. You will miss some story bits though if you care about those.
Obsidian can be gotten in the tunnels, either as drops from the fights, found in chests, or given as gifts for saving people (or you can cheat).
so if i understand correctly, the outcome of the route in each book is completely self-conclusing and doesn't have any effect on the outcome of the previous/following book, uh?
Basically yeah. If you care for story and gameplay progression, it's still best to play them in order but if you know the gist of the previous book, you can theoretically, start any book you so desire.
If you search though the thread, you will see most of the art packs posted. If you find the most recent one (21 was the last one I saw), it has a file with links to the other 20), it contains the gifs, and everything else.
