Amazon on Tuesday released software for buying and reading digital books on a PC and synchronizing the downloads with other devices, including Amazon's Kindle electronic reader

The Kindle for PC application is Amazon's latest attempt to attract new customers and hold on to existing ones through expanded portability of its e-books. Amazon is under increasing pressure from rivals, particularly Barnes & Noble, which is taking pre-orders for its Nook reader announced last month. That device is scheduled for release at the end of the month.

With the latest software, e-books purchased from Amazon can now be read on a PC, Apple iPhone, iPod Touch, as well as the Kindle and Kindle DX. Amazon has promised to have a version of the Kindle software for Apple Macs soon.

Like the iPhone and iPod Touch apps, Kindle for PC doesn't require the person to own a Kindle. However, if a person is reading Amazon-purchased e-books on multiple devices, the software will synchronize bookmarks and last page read and allow the viewing of notes and highlights made on any of the devices.

In addition, Kindle for PC displays e-book graphics in color and can take advantage of the touchscreen capabilities in Windows 7, making it possible to zoom in and out of text with a pinch of the fingers. In the future, people will also be able to turn pages with the swipe of a finger.

Amazon's online bookstore offers 360,000 e-books, including most New York Times Best Sellers. That's roughly a third of the books Barnes & Noble plans to make available for the Nook.

A weakness in the Kindle is Amazon's use of proprietary copyright protection technology, instead of the open ePub standard used in the Nook and Sony's Reader, some analysts say. Use of the standard provides access to more content, such as the half-million non-copyrighted books available through libraries on Google Books.

Amazon currently leads the nascent e-reader market, accounting for about 70% of sales, according to Forrester Research. Barnes & Noble, however, could be a formidable challenger. Pre-orders for the Nook have been so strong that the bookseller has told customers that new orders won't ship until Dec. 11.

Sales of e-readers will reach 3 million units this year, according to Forrester. Sales next year could exceed 6 million units.

InformationWeek Analytics outlines the 10 questions you need to ask to see where netbooks fit within your organization. Download the report here (registration required).