In the Windows Phone 8 the new OneNote app shows a list of notebooks categorized by "opened" and "closed".
You can choose one opened and close it. To open a closed one just tap like you does with opened.
So why does this categorization exists?
An Open notebook allows for syncing to keep everything up to date on the device. If you "close" the notebook, then changes to that notebook won't take effect on your phone until you open it again.