This carefully crafted ebook is formatted for your eReader with a functional and detailed table of contents. The Focus On books are made out of collections of Wikipedia articles regrouping the most informative and popular articles about a specific subject. The Focus On books are a result of a substantial editorial work of selecting and grouping relevant articles together in order to create a valuable source of information about specific subjects. This book does not contain tables, illustrations or illustration descriptions. Focus On (an imprint of OK Publishing) charges for the convenience service of formatting these e-books. We donate a part of our net income after taxes to the Wikimedia Foundation from the sales of all e-books based on Wikipedia content. You can access the original Wikipedia articles on the internet free of charge.