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.