http://www.apple.com/ibooks-author/
Currently, the requirements to run iBooks author is OS X Lion (10.7). However, be aware that OS X Mountain Lion (10.
will be released next week, and after that, expect future iBooks author updates to require the newesst OS X version:http://www.apple.com/osx/
Be aware that if you have an older Mac or Macbook (2008 or older) with Intel graphics chipset, it may not be compatible with OS X 10.8. So if you want to play with iBooks author, get it now.
If you don't own a Mac and want to buy an used Mac, I'd recommend making sure that whatever you buy is compatible with OS X 10.8. It's not difficult to score a 2009+ Macbook or Macbook Air with 1.6 ghz CPU for ~$500 used on craiglist.
Lynda.com has recently offered an iBooks author course. You can watch a few of the training videos for free:
http://www.lynda.com/iBooks-tutorials/i ... 460-2.html