Man's Search for Meaning
by Viktor E. Frankl
This book was by far the most common answer in a thread I saw on Twitter about books on happiness. Based on Frankl's survival of Auschwitz, it's more intense than I had assumed. It's a great book, and I recommend it wholeheartedly.
tl;dr: "Those who have a 'why' to live, can bear with almost any 'how.'"