... But use Google's secret for advertising; show relevant ads and enhance the experience.
Advertise later books in the series and similar books. I can also see very useful similar product ads. For example, with a book set in some locale carry an ad for trips to that locale. It would really be interesting to see how to actually go to the places named in the book. Or advertise the products used.
Of course, the problem will be when tailor-made adverbooks are written. Books just to promote a place or product. I can remember the low quality of some kids books that were solely created to push a brand.
To me this is the relationship between real web sites in google results and content farms that have a veneer of content,nbut no real substance. Genuine books with genuine tie-ins would be interesting. Artificial ad-vehicles not so much.