An excellent book on depression, arguing it is an evolved capacity of the mind:

  http://psychcentral.com/lib/the-depths-the-evolutionary-origins-of-the-depression-epidemic/00018693

"Overall, this is a surprisingly accessible book and one that would serve any layperson well as an introduction to the science of depression. Rottenberg’s practical style and talent for using real-world examples by real-world people to illustrate states of low and high mood is refreshing. While not an in-depth tome by any means on depression, treatment, or evolutionary origin, the book is a wonderful first step for those who wish to better understand the illness from a scientific viewpoint. And it gives the reader hope by suggesting that depression is a common, albeit painful, human experience: that a low mood does not mean we have failed."