No doubt that’s possible when books go digital, but Amazon is not taking that full step yet. Rather the lower-priced Kindle (that is already quite inexpensive) will show ads on the home screen and when the reader is switched off.
Amazon to Sell Cheaper Kindle Supported by Ads – NYTimes.com
It would be an interesting experiment to show context-based ads, both based on the text as well as the external situation. For instance, if you’ve read continuously for hours (that would be easy to detect by measuring how often and steadily you flip pages), it could show ads for soft drinks and food, or sleeping pills (provided it’s night).