Big, bigger, but not yet biggest: According to the Association of American Publishers's StatShot study released Thursday, e-books made up nearly a quarter (22.55%, to be precise) of U.S. trade publishers' book sales last year. That's up from 17% in 2011 and just 3% in 2009.

The financial outlook for books is good. Total industry revenues increased 6.2% to $7.1 billion, of which $1.54 billion came from e-books. Net revenue from adult fiction/non-fiction and children's/young adult books rose as well — though revenue in the third category, religious presses, did not.

It wasn't just e-books that sold well. Downloadable audiobooks and paperbacks also saw year-over-year increases, as did hardcover copies of books in the children's/young adult category.

Interestingly, e-book revenues appear to be outpacing user adoption of the format, albeit only slightly. According to a study published by Pew last week, 21% of American adults have read an e-book in the past year.

Image courtesy Flickr, TheCreativePenn