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.

On Thursday, Sony will provide a software update to 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.