The Power of Negative Emotion is a refreshing reality check against the constant exhortations to be mindful and think positively.
Mindfulness, kindness, and positivity can take us far. But they cannot take us all the way. Sometimes, they can even hold us back. Emotions like anger, anxiety, or doubt might be uncomfortable, but it turns out that they are also incredibly useful. Kashdan and Biswas-Diener explain the importance of developing our "emotional agility" - the ability to access our full range of emotions (not just the "good" ones).