The Rockefeller University Press is offering,
free,
The Hostage Brain, by Bruce S. McEwen and Harold M. Schmeck, Jr. It's 323 pages.
For this book, you can choose your favorite digital flavor: pdf, ePbub, and/or Kindle (mobi). You can even get all three, if you'd like (I tried, and it works).
Here are a couple of book descriptions from the book's webpage:
The Hostage Brain illuminates for the lay audience--through clear prose and striking illustrations--what the brain is, how it functions, and how it is often subverted.
and
Many people are vaguely aware of the power of the human brain but few are aware of its vulnerability to quirks of temperature, chemistry, infection, and physical impact. The human brain can be captured by the flood of hormones it sends pulsing through the body or taken hostage by outside events and even its own thoughts. The Hostage Brain illuminates for the lay audience--through clear prose and striking illustrations--what the brain is, how it functions, and how it is often subverted. It reveals the most recent advances in brain research and describes key discoveries together with the social and political implications that surround them.
Unfortunately, there are not enough ratings at Amazon (just one) or GoodReads (none) to get a good read (no pun intended) on the quality of this book. The only other book by author McEwen that I could find gets high ratings at both Amazon and GoodReads, however.
This is a very valuable deal. The book is apparently not available as an ebook at Amazon or anywhere else. Secondly, there is a good little cost for a physical copy. Both the retail price and the price at Amazon for the hardcover is $19.99. You can get a new paperback copy for $13.30 at B&N.
I suppose that you would like to have a link to the book's webpage at Rockefeller University Press? Here it is:
http://books.rupress.org/catalog/book/hostage-brain.