For me, it was "Man's Search for Meaning" by Victor Frankl.
When I first encountered Frankl's words, I was struck by how he found meaning in the most unimaginable circumstances of the concentration camps. There's this moment he describes, standing in the freezing dawn, where he thinks of his wife's face and realizes that love transcends physical existence: that no one can take away our ability to choose how we respond to our circumstances.
That passage transformed how I think about suffering and purpose. While my own challenges pale in comparison, his insight that we can find meaning even when everything is stripped away has stayed with me. His concept of "tragic optimism", the ability to remain hopeful while acknowledging life's inevitable tragedies - offers a profound framework for facing adversity. When I feel overwhelmed, I often return to his core message: those who have a 'why' to live can bear almost any 'how'.