It's evidence that e-books (whether for Kindle, Nook, tablet computing devices or any other device you might wish to use to read many thousands of words) are starting to become a more mature technology. They seem to be through their explosive growth phase.
It was inevitable, of course. The question was always "at what share of the book market will e-books settle," not "when will print books cease to exist." Old technologies never die, they just fade into a smaller, niche offering; television supplanted radio as the dominant mass medium in the middle of the last century, for example, but radio is still a big business.
But the fact that that leveling off is already happening with e-books suggests that the ratio of printed books sold to electronic books is going to stabilize at a higher level than it had seemed likely a year or two ago in the era of extraordinary e-book growth. Carr has a number of good ideas for why; I find his first most compelling.
We may be discovering that e-books are well suited to some types of books (like genre fiction) but not well suited to other types (like nonfiction and literary fiction) and are well suited to certain reading situations (plane trips) but less well suited to others (lying on the couch at home). The e-book may turn out to be more a complement to the printed book, as audiobooks have long been, rather than an outright substitute.
Let me phrase it a different way: If you're someone who reads a book every week during your commute, say a detective novel or romance novel, the e-book format is perfect for you. But those people have pretty much all shifted to e-books, and there are only so many of them. If you read a book of serious nonfiction a month at home, and maybe even put it on your shelf afterward as a bit of a trophy, printed books are pretty darn good.
And for publishers (and authors, which, in the interest of disclosure, I should note that I am) what matters most is not the competition between e-books and traditional books. What matters is how much of people's total time they are willing to devote to reading, as opposed to the overabundance of other entertainment options that the world offers.