On the Area 51 page it show the statistics on the Ebooks beta site
As were are not public yet, the recommended values are probably not valid (5 users with 3000+ rep e.g.), but it is interesting to keep in mind.
I was specifically wondering about the values in the box on the middle right in the "Beta" tab.
committed users 221 users committed 56.6% signed up for beta 0% fulfilled commitment
What can I do to fulfil (my part of) the commitment and change the zero in something more positive?