I am not a big one for self-help books. Like diets, their benefits seem to fade quickly due to the superficiality of the effect.
However, I have heard good things about Barbara Frederickson's 2009 Positivity: Groundbreaking Research Reveals How to Embrace the Hidden Strength of Positive Emotions, Overcome Negativity, and Thrive.
We all want to thrive and flourish in life, as best we can. Why not? Life is short. A quote from the Amazon review:
"Positivity is literally the feel-good book of the year, providing a scientifically sound prescription for joy, health, and creativity. Read one to two chapters daily as needed or until grumpiness subsides."
—Daniel Gilbert, professor of psychology, Harvard University, and author of Stumbling on Happiness