Amazon's Kindle for iPhone is a free application for the Apple iPhone, Apple iPhone 3G, and Apple iPod touch that allows these devices to access any books purchased from the Amazon Kindle Store, effectively bringing this content that was originally exclusive to the Amazon Kindle and Kindle 2 lines of ebook readers to Apple's devices. The killer feature of the software is the Whispersync feature that allows owners of multiple Kindle-compatible devices to synchronize the books they are reading between their Kindle and their iPhone and iPod touch. This way, if you stop reading on page 57 on your Kindle then later pick up your iPhone, but the last page you had read there was page 27, you can simply sync wirelessly with Amazon's servers and the application will bring you back up to page 57.

Obsessable has a full review of the application that notes some of its limitations. For example, one cannot purchase items directly from the Amazon Kindle Store via the application, there is no multi-touch zoom-in and zoom-out capabilities so reading items like graphic novels and manga on the device is impossible  (see update below), and Amazon does not offer any of the magazine or newspaper subscriptions available for the Kindle on Kindle for iPhone.

UPDATE: On May 20, 2009, Amazon released an update to the Kindle for iPhone application that added the ability to view books in landscape mode, lock the viewing in landscape mode so that turning the iPhone wouldn't suddenly reposition the page, and multi-touch zoom capabilities for any pictures within books.