In an attempt to expand the usefulness of the Kindle against rising competition, Amazon on Thursday unveiled a software development kit for building applications for the electronic reader.

Along with the SDK, Amazon will let developers sell their applications through the Kindle Store, using a self-service publishing platform. Amazon expects to start offering applications sometime this year.

Amazon made the announcement as competition grows beyond other e-readers, such as Barnes & Noble's Nook and the Sony Reader. Indeed, far more versatile mobile devices suitable for reading are becoming available at prices competitive with e-readers. Those devices include smartphones, smartbooks, mobile Internet devices, and tablet PCs.

Industry observers expect Apple to introduce a tablet PC at a San Francisco news event scheduled for next week. The computer maker has only said it would unveil its "latest creation." If such a device exists, it would likely support digital books, as well as music and video, analysts say.

With the Kindle Development Kit, Amazon is likely trying to avoid obsolescence of its relatively young product by letting third-party developers expand its usefulness. The kit includes sample code, documentation, and an app-testing Kindle simulator that runs on a Mac OS X, a Windows PC, or Linux desktop.

"The Kindle Development Kit opens many possibilities -- we look forward to being surprised by what developers invent," Ian Freed, VP of the Kindle, said in a statement.

Amazon is releasing a beta version of the SDK to a limited number of developers. Those who sign up for the program will be notified next month whether they were chosen.

A few companies are already building Kindle applications. Handmark is developing a Zagat guide featuring ratings and reviews of restaurants in cities worldwide, and Sonic Boom is building word games and puzzles. Video game maker EA is also planning to offer games for the Kindle, but has not offered any details.

Developers targeting the Kindle will face some restrictions, particularly in the use of the device's Whispernet wireless connection, which Amazon offers at no charge to customers for buying books directly from the online retailer.

Applications that are smaller than 1 MB and use less than 100 KB of wireless data per user per month can tap Whispernet at no charge. More than 100 KB, and Amazon charges 15 cents per MB.

Other limitations include a black-and-white display and the inability to show video or complex animation. Pluses include long battery life and an always-on wireless connection.

Amazon is offering developers a revenue split of 30% to Amazon and 70% to the developer.