NEW YORK — With the market for electronic books still relatively sleepy, Sony Corp. is trying a new tack: untethering the latest model of its e-book reading device from its own online bookstore.

Today, Sony will provide a software update for the Reader, a thin slab with a 6-inch screen, so the device can display books encoded in a format being adopted by several large publishers. That means Reader owners will be able to buy electronic books from stores other than Sony's.

"This upgrade opens the door to a whole host of paid and free content from third-party e-book stores, Web sites and even public libraries," said Steve Haber, senior vice president of consumer product marketing for Sony Electronics.

With the move, Sony is partly letting go of its e-book business model, under which it sold the $300 device and the books that could be read on it. It's also a challenge to www.Amazon.com Inc., which last year put out its own e-book reader, the Kindle, and tied it to its own online store. Amazon, however, makes it relatively easy for publishers and individuals to submit books to sell through the store, with Amazon taking 65 percent of the proceeds.

Opening up the Reader could also help Sony catch up to the $359 Kindle in terms of book selection — Sony's store, which it will keep running, has about 45,000 books available, while Amazon's Kindle store sports more than 140,000.

Sony's move could also help energize the e-book industry, which has yet to take off, despite the investment of big-name companies like Sony and Amazon. Neither has released sales figures for their reading devices.

International Digital Publishing Forum, the main e-book publishing trade group, said e-book sales by a dozen major U.S. publishers amounted to $31.8 million last year at the wholesale level.

The publishing forum backs the Epub format that the latest Reader model will be able to handle after the upgrade. Publishers supporting Epub include Simon & Schuster, Penguin Group, HarperMedia, Hachette Book Group, HarperMedia and Harlequin Enterprises Ltd. Epub allows the supplier to copy-protect a book, to prevent piracy.