101. Think and Grow Rich - Napoleon Hill

This classic business and self-improvement book was first published in 1937, and is arguably one of the most famous self-improvement books today (more than 30 million copies have been sold). It is based on author Napoleon Hill's prior work, The Law of Success, which was the result of research on Hill's close association with people who achieved great wealth during their lifetimes like Thomas Edison, Alexander Graham Bell, Henry Ford, John D. Rockefeller, and Charles Schwab. Apparently, Andrew Carnegie asked Hill to study the characteristics of these men, and from this Hill developed 15 laws of success and 13 principles of personal achievement.
Some of Hill's "laws" are admittedly a bit "obvious": desire, faith, persistence, etc. But the anecdotes from men like those mentioned above certainly do make this a great read. Also, Hill combines these laws with concrete, practical steps by which by which "desire for riches can be transmuted into its financial equivalent."
One of my favorite chapters discusses the "six basic fears" that are centered around negative emotions and can hold you back from achieving success. They are: the fear of poverty; the fear of criticism; the fear of ill health; the fear of loss of love; the fear of old age; and the fear of death.
There's also a great chapter called "The Mystery of Sex Transmutation." From that chapter:

Sex, alone, is a mighty urge to action, but its forces are like a cyclone--they are often uncontrollable. When the emotion of love begins to mix itself with the emotion of sex, the result is calmness of purpose, poise, accuracy of judgment, and balance .... When driven by his desire to please a woman, based solely upon the emotion of sex, a man may be, and usually is, capable of great achievement, but his actions may be disorganized, distorted, and totally destructive. When driven by his desire to please a woman, based upon the motive of sex alone, a man may steal, cheat, and even commit murder. But when the emotion of LOVE is mixed with the emotion of sex, that same man will guide his actions with more sanity, balance, and reason.

After I read this book, I realized that all other business/self-improvement books I've read get their wisdom from this one. So instead of reading all the other ones that are out there, just check out this must-read classic.