The Sno-Isle system in Washington state (
http://overdrive-audio-books.sno-isle.org ) has only been carrying ebooks for a short time (maybe 6 months), but they're building their collection up extremely quickly - as of today 4,475 ePUB and 2,056 PDF. I know there is some overlap and I haven't figured out a way to get exact numbers, but the best estimate I can come up with is that about 20% of the PDF titles are also available in ePUB. If you deduct that 20% from the PDF number, the total is still approximately 6,100 unique titles.