Book excerpt: This book discusses entropy and the Second Law of Thermodynamics in such a way that everyone can understand its subject matter. Entropy is one of the most interesting concepts in physics. Although it is a well-defined concept, it is still perceived by even well-known scientists as a concept cloaked in mystery. It is also the most misused, and often abused, concept in physics. In order to understand entropy, one needs to understand the Shannon measure of information, and in order to grasp this idea, one must be familiar with some basic concepts of probability. Therefore, this book consists of three chapters: the first discusses probability, the second addresses Information Theory, and the third considers entropy and the Second Law of Thermodynamics. Readers will discover that the Second Law is nothing but a law of probability.