Amazon Kindle users can manually update the software of their e-readers to version 2.5, and start Tweeting. Software upgrade includes several other important features, such as the ability to pan & zoom PDFs, and organize content into folders, free social network connection.
After installing update, you must link Kindle to Twitter or Facebook in Kindle's experimental browser. Once finished, you can select text from within your e-books, and share them to social networks. With text selections, you're given a little space for an associated message. The text selection then sent out as a link to a page hosted by Amazon.
You can simply add a text annotation to your book and share that, just start typing and hit "save and share."
Here's an example of a plain Tweet from Kindle. As you can see, that text is also presented with a link and hashtag, so your space is limited. But since the Kindle has a free connection to Sprint's 3G network, it's easy to overlook such a shortcoming when you can freely send updates wherever there's coverage.
Download: Kindle Software