Man's Search for Meaning
By Viktor Frankl

I read this memoir in my early 20s—it was more satisfying than the godless existential work I'd read and found companionship in. This is a diary of Frankl's struggle to make sense of the enormous suffering he witnessed as a prisoner in a concentration camp. He comes to believe that even while enduring horrific experiences, we can maintain our humanity. In the second part of the book, he presents his theory of logotherapy, the idea that if we can find meaning in an experience then we can endure any psychic suffering. It's one of the most significant books of the 20th century.