The Intelligent Investor by Benjamin Graham
It is a must read if you need to understand investing. As this book was originally written long time ago, commentary by Jason Zweig clarifies and bring up to date some important points. Time goes by but basically things about investing have not changed a lot, and this book explains and demonstrates it in simple ways. In addition to the technical side of the book the most important take away is the clear distinction between an investor and a speculator.
