Correct Posture - Its meaning and its results is a fascinating little book on posture published in the 1930s by a man who appears to have no selfishly-motivated interest in sharing his knowledge and his experience (this was the first indication to me that this book was worth reading: he was sharing this knowledge because he truly believed in it and saw its truth in those who applied it).
For me the most interesting part of his story is how he came back to the western world with this newfound knowledge after spending some time exploring India and speaking with practitioners of yoga.
The principles presented in this book are incredibly simple and I can tell you, after applying them for less than two days myself, that it's the real thing. Try it and see for yourself. The book is short and you can read the entire thing online for free here.
I'm including a few of my favorite quotes from the book below.
Physical strength is of value to a man or woman only when it becomes a medium for a better and fuller life, a life of constructive labor an achievement.
Muscles are not the outward signs of inward health of body and mind, but they are part of man's equipment to 'carry on' in the rush and tumble of the world.
[...] misunderstanding is the world's worst disease with which man has to contend.
Teachers should themselves be taught first the value of posture and air--then they can turn to teaching it to the children. Make it necessary as their book education.
From the body of Arabian literature comes these words, as true today as when they were first uttered by some scholar in ancient Bagdad or Mecca: "He who has health has hope, and he who has hope has everything."
"The only way for a rich man to be healthy is by exercise and abstinence: to live as though he were poor." - Sir William Temple
"Refuse to be ill. Never tell people you are ill: never own it to yourself. Illness is one of those things which a man should resist at the onset." - Lord Lytton