If you hit a dull patch in the book, can you resist the pull of YouTube, Twitter, or your email? Even if you're engaged in the book, Google may beckon to clarify a point in the book ("Essex? Where's that?") and next thing you know, 25 minutes have elapsed in surfing. Perhaps interesting, perhaps productive, but not what you sat down intending to do.
Many people I've spoken with have the impression that this sort of distraction is predictable, and that it is a greater problem when reading on a tablet computer, even compared to reading a print book with a computer nearby.
The data on this question are still thin, but I do know of one relevant study (Woody et al, 2011). Nearly 300 college students took part, each reading a chapter from an introductory psychology textbook in one of five formats: print textbook, printed text pages, printed manuscript in MS Word, electronic pdf file, or electronic textbook. Some students read in a laboratory, some at home, and everyone took a quiz on the chapter material after reading it.
The results showed that media format did not affect quiz grades. But students who read electronic media versions were more likely to respond to instant messages and email while reading, and were more likely to use social networking sites (Facebook/Myspace) while reading.
It's only one experiment, but this feels like an instance where the intuitions of the majority of people will end up according with data. Whether the extra level of distraction is really a problem remains to be seen; and it may well be that users (or software designers) come up with strategies to solve the problem, if it proves significant.
Woody, W. D., Daniel, D. B., & Stewart, J. M. (2011). Students’ Preferences and Performance Using E-Textbooks and Print Textbooks. In F. Columbus (Ed.), Computers in Education. New York: Nova Publishing.