The Berkman Center's David O'Brien, Urs Gasser, and John Palfrey have just posted a 29-page "briefing paper" on the various models and licenses by which libraries are providing access to e-books.

It's not just facts 'n' stats by any means, but here are some anyway:

According to the 2011 Library Journal E-Book Survey, 82% of libraries currently offer access to e-books, which reflects an increase of 10 percentage points from 2010. … Libraries maintain an average of 4,350 e-book copies in a collection.

[T]he publisher-to-library market across all formats and all libraries (e.g., private, public, governmental, academic, research, etc.) is approximately $1.9B; of this, the market for public libraries is approximately $850M

92% of libraries use OverDrive as their e-book dealer

Of the major publishers, only Random House allows unrestricted lending of e-books.

I found the section on business models to be particularly clarifying.