The market leader in print and audio fiction.
Fiction is ephemeral - mostly read and then discarded. The awesome potential for this service is in factual and reference books that are used hundreds or thousands of time each in their lifetime.
The 1.5 books per month metric is diluted in that case when it's possible to dip into 100 books per month just to extract the one snippet of information needed from each.
Well this is a whole different issue entirely, and could start another entire discussion.