Global Industry Analysts says the worldwide market for online books will be just shy of 11 figures by 2010:
Driven by growing popularity of online shopping, lure of discounts offered by retailers, faster delivery, free shipments, and the convenience offered by home shopping, the market for online books is projected to reach US$9.5 billion by 2010.
It’s important to note that "online book" does not mean "eBook" (though they do say "The emerging ebook concept is a lucrative offshoot of the online book industry"). When they say "online book" they appear to mean "book purchased online," making this a potentially more palatable projection than the infamous Arthur Andersen forecast of a ~$3 billion eBook market by 2004.
(As an aside, that Arthur Andersen link above is the first time for me that the best result from a Google search came from GBS. Somehow I don’t expect it will be the last.)