A serious, in-depth book about happiness, which the author calls positivity to distinguish from the superficial fluffy self-help happiness stuff. She defines positivity to include joy, gratitude, serenity, interest, hope, pride, amusement, inspiration, awe, and love. This book is full of results of scientific studies done by the author and others--everything she says is backed up experimentally.
The biggest take-away here is that a 3:1 ratio of positive to negative thoughts is the tipping point for flourishing. Below that, not much happens.
Positivity spurs renewal; negativity leads to decay. Positivity builds psychological strengths, like optimism, resiliency, openness, acceptance, and purpose; mental habits like mindfulness, awareness, and living in the moment; social connections; and physical health.
The book gives suggestions for reducing your negativity, including disputing negative thoughts, using healthy distractions to stop ruminating, and becoming more mindful. She makes the point that some negativity is good--if you're in a bad situation and need to do something about it, or even just acknowledge the reality of it, you need negativity. However, gratuitous negativity (excessive, redundant, ugly, unhelpful) should be minimized.
It also gives suggestions for increasing your positivity. These include finding positive meaning in your situation, making plans and goals, savoring goodness, doing and noticing kindness, following your passions, dreaming and visualizing about your future, connecting with others, and communing with nature.
The author gives some specific exercises for how to do these things, but they're mostly obvious from the lists above. Overall, an interesting and serious book that taught me a lot. As it happens, I read this right after What Happy People Know, so it was an interesting contrast. They both cover the same topic and make a lot of the same points, but What Happy People Know (also by a scholar/psychologist) is very cheery and doesn't contain a single footnote, whereas this one is all about the science. Fun stuff either way.