130 milllion ebooks have been downloaded from iBooks since it launched last year. Since we also know that as of the iPad 2 launch, it had seen 100 million downloads, it's safe to assume that 30 million ebooks have been downloaded in the last 3 months.
Note that this is downloads, not purchases, but it's still a decent number. In comparison, the main Project Gutenberg website saw 3.9 million downloads in the past month. iBooks had roughly 2 and a half times the traffic. That's not bad. Of course, PG have a far smaller collection of titles (36k on the main site).
P.S. Check back later; I might have some figures on downloads vs purchases.