The fact of the matter is, we are a product of our environment and we naturally try to pick up our traits, values, expressions and emotions from everything and everyone around us. But since books pull you in so much, it's easier to become influenced by them than, say, a short movie. I didn't need OSU to tell me this.
However, one critical point here is that the effect is often temporary. I've also noticed that often, where I will have a sudden boost of inspiration and energy after reading a particularly uplifting book, but this only lasts for days or weeks, depending on the book.
And that, my friends, explains why self-help books sell so much.