I've added a environment texture as my background as well as lighting for a house. It works fine. At one point I could edit the environment texture as well as adjusting hue or brightness in the node editor. But now I couldn't figure out how to bring it back. The node editor only shows the last selected object. And I can't select "world" in the outliner either. Any idea?
You can select world materials in the Node editor by pressing the world icon in the header:
The cube icon will switch back to showing object materials.