In an article from January this year, Kevin Kelly has described an interesting model to pay people to read books in order to motivate them to read from cover to cover. He pointed out that e-book readers could track reading behavior and compare it to the norm of certain metrics to determine whether that person is really reading or just turning pages. He stated the problem that people bought books, but rarely finished them.
Readers are very different, which can make the measurements of the proposed metrics hard enough. A reader can take a book, read a bit, leave it for a month or more, because of temporarily losing interest, and then return to it later when she finds more information to support her interest. Many people like to alternate what they do in order to keep their engagement high. Some people read faster, some slower, some need to reread sentences to fully understand the meaning, some need to highlight things. We can't put all readers under a common denominator so easily.
The danger of a person deciding to "read" the same book on a different device just to get one more dollar means that there must be a central store in the cloud for every person and book on the planet. Who will be so kind to support this?