Amazon.com started yesterday (Monday) shipping the Kindle 2 e-book reader, a day earlier than scheduled.

The Kindle 2 was introduced two weeks ago for preordering and, according to Amazon.com, it is already the site’s No.1 seller. The Kindle 2 was unveiled by Amazon.com CEO Jeff Bezos and best-selling fiction writer Stephen King on February 6 in a New York event.

According to most reviews of the gadget, the Kindle 2 was visibly improved compared to the first version of the e-book reader. The Kindle 2’s combination of new hardware and superior book-buying experience puts it in pole position compared to the competition. The only real rival seems to be the Sony Reader.

The Kindle 2 is available for $359, the same price as the first version, while the Sony Reader sells for $300.

Compared to the original version, the Kindle 2 has several improvements. The buttons are better places so users won’t have the problem of unintentionally turning pages or entering the menu anymore. The Kindle 2 buttons are smaller and better placed and the buttons for turning pages are redesigned and no longer respond to a stray press on the edge of the reader.

The scroll wheel of the Kindle 1 was replaced with a more usual five-way navigation control. Ad a cleaner look and about 50% more slimness and you get a super e-book reading experience.

The Kindle 2 also features 3G wirelesses so users will download their e-books faster. The gadget has 2GB memory which means you could store more than 1,500 books, 7 to 8 times more than you could store on the Kindle 1.

Other interesting features of the Kindle 2 include a text-to-speech feature that allows the device to read aloud to users, a built-in 250,000-word dictionary, and 25% longer battery life.