The one book I wish my colleagues would read and learn from is "Man's Search for Meaning" by Victor Frankl .
"Between stimulus and response lies a space. In that space lie our freedom and power to choose a response. In our response lies our growth and our happiness."
I am a existential therapist who works with suicidal people on a daily basis for over 20 years. Here are a couple of sources I have found to be illuminating.
Man's Search for Meaning by Viktor Frankl Very widely cited but no less moving, written by a psychiatrist who loses everything in the Nazi concentration camps of WWII this book provides a template towards finding personal meaning in context of great suffering.
Less popularly well know is the author, Dr. Irwin Yalom PhD who explains existentialism in context of doing practical therapy. Existential Psychotherapy discusses those aspects of the Existential philosophy as it might apply to everyday life and personal meaning with special focus on death, freedom, aloneness, and meaninglessness. Dr. Yalom is a truly original highly regarded theorist in psychology but also a very accessible writer for layman. This book has richly informed not only my professional practice but also my personal life. Plus he cracks a bit on Freudian Psychoanalysis and Skinner behavioral theory, always a fun sport IMHO!
Man's Search for Meaning is a good read for those interested in the topic.