How do I find out what my iBooks "authorized computers" are?
The iBooks authorization count is the same count you get for iTunes Store. They're the same thing. This covers Books, Music, Movies, etc... If you go to the Store menu item from iTunes or iBooks you can click View My Apple ID, enter your password and then you'll see the authorization count.

Basically if you have authorized another computer to use iTunes Store it counts as a computer authorized for that Apple ID. That's why you're seeing the count as 2. It's no longer just related to music. As Apple's info states on the Account Information screen:


  2 computers are authorized to use content purchased with this Apple ID


If you want to reset this, just click the Deauthorize All button and you can re-authorize your 1 computer to start fresh. You won't be able to see what computer it is, but you can see the count and reset it.