I don’t know how long this book will be available for free but I just downloaded the Kindle version of “Social Engineer: Would you trust a computer hacker?” by Ian Sutherland.
UPDATE: In the comment section you will see Ian Sutherland plans to keep this book free permanently. That’s great news. You’ll also see why it’s free.
It looks like there are free versions (electronic only I think) available from Kindle, Kobo, Nook, Scribd, Smashwords, Amazon.com. Barnes & Noble, Google Play, iBooks, and iTunes.
Truthfully, I just sent it too wait in line on my Kindle so I don’t have a book review. It’s recent (August 1, 2014), it’s short (88 pages), and has 3.8 out of 5 stars (26 ratings so far).