Here's a very nicely done video on the last steps on publishing your iBooks after you are done with iBooks Author. It details out the steps in depth and even shows you a web site to buy a ISBN number, which can cost a lot from what Apple suggests. The website it suggests is epubbud.com where you can get an ISBN for $5.
As mentioned in the video, please do take time to read the Apple contract and license agreement carefully. Your iBook will be selling on Apple's bookstore for free or for a price ranging from $.99- $39.99. Even if you are giving your book away free, you still will be required to go through the entire procedure as if you were selling your book for a fee.
