Right now on Amazon you can get Twitter for Good: Change the World One Tweet at a Time. This book will only be available for free today (September 6, 2011).
This book is also available for free at Barnes & Noble.
See a huge list of more free downloads here. These books can be downloaded for free to your computer, phone or Kindle. Be sure to check that the price is still free before ordering, as prices fluctuate frequently on Amazon.