From The Mac Observer :
Apple said on Tuesday that the company has had 180 million downloads from its iBooks ePub book store. The news came during Apple’s quarterly conference call  with analysts, and served to highlight Apple’s ongoing success with iTunes. …
Apple didn’t offer a dollar amount for its iBooks sales , nor did it quantify the number of units sold versus the number of units downloaded. Some books are downloaded to multiple devices per account, and some of the titles available on iBooks are free. Apple has never specified what it determines as a “download” from the many facets of its iTunes store.
We noted, however, that Apple also said it has now sold 40 million iPads. 180 million iBooks downloads means that the company is so far averaging 4.5 iBooks per iPad sold, though those numbers are skewed by several factors.
More in the article.