In 2008, the Princeton University Press first released a book formatted for a wireless e-reader. Today, about 90 percent of the 240 books that the Press releases each year are available for consumers to read on their Kindles, iPads, Nooks, and other electronic devices.

“Two years ago we were saying that the retail e-book market had arrived,” said Priscilla Treadwell, the Press’s electronic publications marketing manager. “It has really arrived now.”

While digital sales make up only 7 percent of PUP’s revenues, they have risen 200 percent in a year, Treadwell said, and the Press expects the numbers to increase significantly as more tablets become available and more titles are available electronically.

Print and electronic versions of a book title are sold at the same price. Highly illustrated books that will not display properly on a tablet are published exclusively in print, Treadwell said. The Press also is converting older releases and out-of-print books into electronic form.

In a digital venture called Princeton Shorts, the PUP in November released five “shorts” from previously published books, priced from $.99 to $4.99. Among the excerpts are “On Reading” from Henry David Thoreau’s Walden and “The Future of Fossil Fuels” from Hubbert’s Peak: The Impending World Oil Shortage, by Kenneth Deffeyes *59, emeritus professor of geosciences. The Press plans to release five more “shorts” by the spring.