As a man thinketh by James Allen
This was a book published in 1903, and yet the contents are still relevant nowadays. The author shared his result of meditation and experience about thoughts in this little book concisely. You can finish it within an hour.
After reading it, I understand why this book inspires so many self-help book authors. James Allen condensed the cause-effect relationship of thought and character, circumstances, health, purpose for us. We are the master of our own life. No one could help you unless you decided to change.