It takes the average reader just seven hours to read the final book in Suzanne Collins's "Hunger Games" trilogy on the Kobo e-reader—about 57 pages an hour. Nearly 18,000 Kindle readers have highlighted the same line from the second book in the series: "Because sometimes things happen to people and they're not equipped to deal with them." And on Barnes & Noble's Nook, the first thing that most readers do upon finishing the first "Hunger Games" book is to download the next one.

In the past, publishers and authors had no way of knowing what happens when a reader sits down with a book. Does the reader quit after three pages, or finish it in a single sitting? Do most readers skip over the introduction, or read it closely, underlining passages and scrawling notes in the margins? Now, e-books are providing a glimpse into the story behind the sales figures, revealing not only how many people buy particular books, but how intensely they read them.

For centuries, reading has largely been a solitary and private act, an intimate exchange between the reader and the words on the page. But the rise of digital books has prompted a profound shift in the way we read, transforming the activity into something measurable and quasi-public.

The major new players in e-book publishing—Amazon, Apple and Google—can easily track how far readers are getting in books, how long they spend reading them and which search terms they use to find books. Book apps for tablets like the iPad, Kindle Fire and Nook record how many times readers open the app and how much time they spend reading. Retailers and some publishers are beginning to sift through the data, gaining unprecedented insight into how people engage with books.

Publishing has lagged far behind the rest of the entertainment industry when it comes to measuring consumers' tastes and habits. TV producers relentlessly test new shows through focus groups; movie studios run films through a battery of tests and retool them based on viewers' reactions. But in publishing, reader satisfaction has largely been gauged by sales data and reviews—metrics that offer a postmortem measure of success but can't shape or predict a hit. That's beginning to change as publishers and booksellers start to embrace big data, and more tech companies turn their sights on publishing.