Navigation Rules review:5 stars (Navigation Rules) - Outstanding. A great help when you're returning to the boating world after many years. A good refresher. Would recommend anybody new or old to the world of boats to strongly consider this.2 stars (Essential for Every Boat Owner, but only a starting point) - The official navigation rules are something that every boater should own. If your vessel is large enough or operating in international waters, you are legally required to have it aboard and understand its contents. Regardless of whether you have to have it aboard, if you go out on the water you have to understand what the rules are and what your obligations are to avoid collisions.
Think you don't need it? Collisions are the most serious boating accident, and virtually every collision is later determined to be avoidable by later investigation, had the operators been closely following the Nav Rules.
Here's the problem, though, if you rely on the Official Navigation Rules as your only reference: this edition of the rules is technical, and illustrated, but is often incomprehensible. The diagrams are schematic by necessity, but they are not useful if you're trying to decide what to do out in the real world. New boaters will be easily confused by the situations, since there's no perspective on what is a typical or atypical scenario. Experienced boaters will be perplexed by the eccentricities of the wording. It is not a tutorial on the rules, and as a reference is decipherable basically if you already know the rules.
So what are you to do, if you have to know the Navigation Rules? You have to own this book. And then you have to learn the rules. There are numerous other sources for the knowledge. You should have a basic seamanship reference, such as Chapman's or the Annapolis Seamanship book. You should probably also take a boating safety class at your local US Coast Guard Auxiliary or US Power Squadron, both of which cover the navigation rules. If you want to be sure about knowing the rules in detail, there are a number of self-study books (such as for Six-Pack exams) that do a much better job at going through a tutorial.
Buy this book; but to understand the basics, and to get to know the details that may save your life and your boat, make sure you extend that knowledge beyond the bare minimum.
Update: I would now recommend Charlie Wing's "One Minute Guide" to the rules of the road in addition to the official Nav Rules (ISBN 0070710945) for recreational boaters.