This book is essential for every collector. Its importance is not just as a source of coin values. In fact, that is the least useful part of the book for more common series because, for one thing, it generally doesn't evaluate coins over MS63. This is great for earlier more expensive coins. Prices for popular series that have few varieties, on the other hand, are more fluid because those coins are being constantly traded; also, higher grade examples of these coins can be obtained by people with smaller budgets. So, while it's useful as a guide to prices for scarcer coins that are hard to obtain in grades over MS63, for most silver coins, the internet venues (such as Coin Values and the PCGS price guide) are more up to date.
The indispensible value of this book lies in its detailed and mostly illustrated listings for just about every variety of every U.S. coin. If you collect early coins, especially coppers, take this book with you wherever you go to buy or bid on coins. In fact, most auction catalogues will tell you the actual page of this book where the coin being sold can be found. You can't get by without it. Just make sure you get the latest edition (as I write, 2008 is available; I haven't seen it yet, but I ordered it from Amazon).
While useful and informative and, indeed, a must if you deal in coins, this book is quite outdated. Since the price of gold and silver have gone up considerably and change daily, the effect on coins is intense in many cases.
There are several sites online where you can get more up-to-date information. Yet having this handy is a good idea. I like having something near my work area and in my hands so I can quickly look up the information I want. Then I can follow that up by going to one of the Web sites.