I humbly propose very basic citation system that allows users to stipulate sources for answers and questions. The new tables collecting citation information could be queried for very interesting data and even monetized. Before you downvote me for the mortal sin of excess complexity without very much benefit, please consider the following:
For users posting answers, it will be fully optional, and could be neatly integrated into the existing views without adding much complexity.
It will generate a lot of very interesting data for users. For example, it would be possible to tell which books and blogs are mentioned the most often in questions and answers. I think this would be fascinating and would significantly enhance the visitor user experience.
It could be monetized in several different ways. For example, popular book sources could be listed on the right column of relevant questions and tag view. Affiliate links could automatically be generated to amazon by using the Product Advertising API. Of course this will have a lower click value than other targeted advertising but this block could be cycled in where targeted advertising is not performing well. I would be willing to bet that these would have a higher click through rate than the existing targeted advertisements, which may improve impression value.
Yes, I agree with the general sentiment that developers are buying and reading less books. However, a quick look at a rough estimate of the audience profile for stackoverflow.com seems to suggest that most of the visitors would be financially able to buy a technical book if it is recommended properly. I can't think of a better suited platform than stackoverflow.
Any implementation could be rolled out evolutionary fashion and could be very subtle.