As A Man Thinketh
This short book, originally published in 1902, has had a huge impact in the field of personal development. It is regarded as one of the most important books of the new thought era. It's written in such a way that makes it easy to understand the most powerful message you could ever learn.
The introduction of the audiobook summarizes the idea beautifully:
Mind is the Master-power that moulds and makes, And Man is Mind, and evermore he takes
The tool of Thought, and, shaping what he wills, Brings forth a thousand joys, a thousand ills:
He thinks in secret, and it comes to pass: Environment is but his looking-glass.
Public Domain (P)2007 Thinking Stuff LLC