By now, Twitter’s media stereotype as the place you come to share details of your last meal is finally starting to fade, giving way to a growing understanding of its real impact on the world.

And while a lot of attention has been going to Twitter as a tool for marketing and PR, Twitter is also emerging as a powerful tool for social change. From fundraising to pay for a Cambodian student’s tuition to organizing the street protests that marked the Arab Spring, people around the world are using Twitter to advance important causes.

Now there’s a new guide for anyone hoping to make a difference in the world using social media: Twitter for Good: Change the World, One Tweet at A Time, by Claire Diaz Ortiz. (If her name sounds familiar, that may well be because she leads social innovation and philanthropy at Twitter.)

I enjoyed her talk at BlogWorld last year, where she set out her TWEET (Target, Write, Engage, Explore, Track) model of Twitter effectiveness. So I’m looking forward to reading the book, which launched today.

And if you are too, good news. Until midnight tonight, Twitter for Good is Twitter for Good for Free: you can download the electronic version at no cost from AmazonBarnes and Noble and iTunes.

And if you miss that deadline, you can always enter to win a copy on Claire’s blog.