'Four-Dimensionalism' is a great book. Sider defends Four-Dimensionalism with great clarity and force. Four-Dimensionalism holds (roughly) that, just as you have spatial parts--e.g. hands, cells, simples, etc.--you have "temporal" parts. A temporal part is a part of you that exists at a certain time in your existence. A consequence of this view is that, at a particular moment, you don't "wholly" exist, because your existence is spread out in time. (Four-Dimensionalism contrasts with Three-Dimensionalism, which holds that you "wholly exist" at every moment you exist.) What's nice about this book is that it takes a difficult, possibly obscure, view and makes it a joy to read about.
The book begins with an "Introduction." Unlike most Introductions, Sider's is important to read because it begins by setting forth the philosophical method and picture Sider is utilizing. (In fact, at several points in the book, Sider falls back on the points he makes in the Introduction.) The book is divided into six chapters. The first presents the "Four-Dimensionalism" picture, and motivates it by showing its ability to handle certain conceptual problems. The second could stand alone, and is a defense of the "B-theory" of time, which Sider assumes for the remainder of the book. The third attempts to state exactly what the Three/Four-Dimensionalism fuss is about. The final three chapters, which take up the bulk of the book, constitute a defense of Four-Dimensionalism--the first two provide arguments for the view, the last defends it against objections. (The book also contains a sizable list of references.)
Sider's writing is clear and easy to follow. At points, things get difficult, and some knowledge of physics is helpful for several sections; but on a whole, a limited amount of specialized knowledge is sufficient to appreciate this book. One thing I liked about it was Sider's approach. He argues that--when all the votes are in--Four-Dimensionalism is the best view, which allows him to be frank about the various strengths of arguments for and against his view. Many of his arguments are persuasive, and he's fairly systematic, considering many different objections. In the end, I learned much from it and would recommend it to anyone interested in metaphysics, ontology, or philosophy of time.