This pocket reference summarises the basics of CVS. It's enough to get you going, but I wouldn't rely on it for a major project. The explanation of how CVS works is short; the part I've found most useful is the handy table of CVS commands and options in the second half of the book.
However, there is extensive online documentation for CVS that goes into far greater depth than this book, is more up to date, and is free. Even though I've had the book, I've still needed to refer to this online documentation to learn the finer points of tags, branching, and other CVS features.
So it's really a matter of whether you want to pay to have some (but not all) useful information in a handy booklet. If that appeals to you, great, this isn't a bad book. But you can certainly live without it by using your computer as a reference tool.