To be sure, this growth varied by genre, noted Bowker’s Kelly Gallagher at Digital Book World this morning. (Bowker conducted this research with the Book Industry Study Group.) E-books now make up 26 percent of adult fiction purchases, compared to 11 percent of children’s book purchases and 3 percent of cookbook purchases.

Seventy-four percent of book buyers have never bought an e-book (and 14 percent of those actually own an e-reader or tablet but choose not to use it to read e-books).

Bowker looked at the habits of “power buyers”—people who purchase four or more print or e-books per month. “Whether print or digital, publishers’ best customers now look alike,” said Gallagher.

Print power buyers make up 22 percent of the overall print book-buying population, and they drive 53 percent of print book purchases overall.

Meanwhile, e-book power buyers make up 35 percent of the overall e-book buying population, but they drive 60 percent of overall e-book purchases. In other words, about a third of the overall buyers drive two-thirds of overall purchases. Casual e-book buyers “are not pulling their weight” compared with casual print book buyers, Gallagher said.

There’s a bright spot for e-book growth: Around 7 to 12 months after buying their first e-book, 72 percent of power buyers switch over to e-books exclusively.

Overall, though, print power buyers slowed their entry into the e-book marketplace in 2011. “That’s why we didn’t see exponential growth,” Gallagher said. And beyond those power-buying voracious readers, casual e-reading is at risk: As more people buy tablets, e-reading becomes just one option among many.

The Digital Book World conference is taking place in New York this week. See all our Digital Book World coverage here.