You can pay per book (and make unlimited changes to that book once you've created it once, if that makes sense) or buy the unlimited version.
To play with it, you don't have to pay anything.
I started out paying per book, and then quickly realised I'd be better off getting the unlimited version because of future publishing plans.
Also, I recently exported my vellum doc to rtf and used that to format for a Createspace paperback. So easy! No more double spaces, no more missing page breaks. Awesome!