[If you're looking for information about the history of mathematics, see mathematics history.]
Note: This review is for the second edition of the book. A newer edition (the third edition) is now available as of 2011.
A History of Mathematics by Carl B. Boyer, revised by Uta C. Merzbach (John Wiley & Sons, 1991) is probably the most definitive reference book about the history of mathematics, starting from ancient Egypt until the present day. Boyer has managed to arrange the content in a way that minimizes jumping back and forth both chronologically and in terms of topics, which makes for a history that flows well.
For the most part, this book is a history of the western mathematical tradition. Indian and Chinese mathematics take a total of one chapter (expanded to two in the third edition). Mesoamerican mathematics is entirely absent. However, the book does an excellent job of coverage of western history, from ancient Egypt to the twentieth century. Coverage of most eras is quite detailed, although a mere seventeen pages are devoted to the twentieth century, which leaves room for little more than a very brief overview of general trends. No doubt it can be difficult for the historian to evaluate the significance of discoveries without the benefit of enough time, and the book is already 714 pages as it is, but this omission seems to be a slight detriment to the book.
Boyer does a good job describing the advances and achievements of the mathematicians found in the pages of history. Those with a mathematical bent might wish to see more equations and diagrams, while those without might find daunting what equations and diagrams are already there, so perhaps on balance the amount of equations is appropriate for a work of this type.
Overall, a very good reference work for those interested in mathematics history.