Amazon is opening its Kindle ebook reader to third-party software apps as part of an apparent effort to fend off an imminent challenge from Apple's tablet.

Late Wednesday evening Pacific time, Amazon announced that it will offer a Kindle SDK (software development kit) sometime next month. The initial release will be billed as a beta, and it will only be available to a limited number of developers.

"We've heard from lots of developers over the past two years who are excited to build on top of Kindle," read a canned statement from Ian Freed, the Amazon vice president who oversees the Kindle. "The Kindle Development Kit opens many possibilities - we look forward to being surprised by what developers invent."

The kit includes sample code, documentation, and a Kindle simulator, which mimics the 6-inch Kindle and the 9.7-inch Kindle DX on Mac, PC, and Linux desktops. Completed apps can then be uploaded to Amazon's online Kindle Store, which currently offers access to books, magazines, newspapers, and blogs. Apps can be offered up for free, sold for a one-time fee, or distributed with a monthly subscription.

Free apps must be smaller than 1MB and use less than 100KB per user per month of wireless data. One-time-fee apps can larger, but they're also limited to 100KB of wireless content per user per month.

All applications must be smaller than 100MB, and any larger than 10MB will not be delivered wirelessly. They must be downloaded from the Kindle Store to a PC and transferred to the Kindle via USB.

Amazon will not permit voice over IP, advertising, "offensive materials," the collection of customer information without express customer knowledge and consent, or the use of the Amazon or Kindle names. The company also says that content must "meet all Amazon technical requirements," must not act like a generic ereader, and must not contain malicious code.

Kindle owners will be able to download apps from the store "later this year." Developers get 70 per cent of any revenue from their apps, with Amazon pocketing the remaining 30 per cent. Before the split, Amazon will subtract $0.15 per MB for delivery over its wireless network

An Amazon spokesman declined to say how long the beta would run or how many developers would have access to the beta. Asked if today's SDK announcement was an effort to steal some thunder from Apple's tablet - an ereader expected to be unveiled next week - he said: "No, we’ve been working on this for quite some time." But you can judge for yourself.

Amazon has pushed out a flurry of Kindle announcements in recent days. The company recently opened the Kindle Store to publishers across the globe, and just hours before announced the SDK, it upped the revenue cut for book publishers. In certain situations, publishers now get a 70 per cent cut as well. ®