A $10 pledge will get you a PDF-version of the book sent via e-mail to your inbox. The benefit of an e-version of a book (whether PDF or Kindle) is the ability to verify sources by simply clicking on a provided link, if available. It also doesn't involve shipping charges, so the costs can be kept to a bare minimum.
A $20 pledge gets you a Kindle version of the book sent to your device via Amazon. The cost is higher because there are higher costs involved in distributing it (Amazon charges per 1 mb); and Kindle is better from a presentation and handling standpoint than a PDF file.
A paperback copy plus a choice of PDF-file or a Kindle edition emailed to your inbox. Probably, the better value reward as you get a hard copy (personally, I'd rather flip a page than keep scrolling down a screen) plus a soft copy for easier reference verification and transportability.
This one gets you a hardcover version of the book. The benefit is that this will be a limited edition as no hardcover copies will be sold going forward through major retailers such as Amazon. If you are looking for a limited edition for your collection, this is the option for you.
A hardcover copy plus a choice of PDF-file or a Kindle edition emailed to your inbox. Probably, the best value reward as you get a limited edition of the book plus a soft copy for easier reference verification and transportability.
This pledge gets you a hard cover version plus a choice of PDF-file or a Kindle edition. As a special "thank you", your name will be listed in the "Acknowledgments" section among the major sponsors. Note that there is no additional fee for international backers on this one.
At this point, I'm not really sure what else to add except a sincere 'thank you". Perhaps, in addition to receiving a personally autographed hardcover copy, a choice of soft copy (PDF or Kindle emailed to you) and having your name listed in the "Acknowledgments" section among the major sponsors you will be interested to ask me questions about the Beslan crisis or my book via a one hour conference Skype call.