As a Man Thinketh, by James Allen

I suspect this is the original popular consumption version of the notion of the power of positive thinking.   It is a brief text, almost a magazine article (and free on Kindle).

This text was referenced in Tim Sanders', "Today we are Rich."

There are ideas that sound nearly Buddhist, such as, "Suffering is always the effect of wrong thought in some direction."    And there is an optimism that may have been the root of more popular concepts such as "the secret," (of which I am not a fan) such as, "...beautiful thoughts of all kinds crystallize into habits of grace and kindliness, which solidify into genial and sunny circumstances..."

Certainly worth the read.