Don't worry -- you won't start seeing ads for tattoo parlors while reading Girl with the Dragon Tattoo. Not yet, anyway. But there's some interesting thinking going around the 'net (kicked off by this article in the Wall Street Journal) about how advertising may soon try to conquer the last bastion of entertainment: books. When you go to the movies, you see ads, when you watch TV and browse the Internet, there are ads everywhere. But why don't you see ads while reading a book? (I mean, besides the obvious conclusion that it's annoying and invasive?)
In the past, it's been because the lead time for books is a wild card. Unlike newspapers and movies, books have a longer shelf life, and different readers could revisit the same material over a period of years rather than days or weeks. You'd have to dynamically deliver ads in some way, and you'd need publishers with know-how and insight about their customers in order to sell relevant ads regularly. In short, you'd need e-books, and you'd need a company (says Snarkmarket) like, say, Apple.
Publishers may not have the ability to sell relevant ads to readers, but Apple surely does, especially since it seems to be sweeping up ad sales people as quickly as possible lately. And with prices becoming competitive in the e-book space, there's incentive for both Apple as an iBook publisher and even authors (who want to supplement e-book sale numbers with iAd sales) to bring advertising over to the iBookstore at some point.