The book is a great compendium of best-practices for building software, traps and pitfalls to avoid.
But most importantly, it is the best source to learn the idioms and patterns used in the .NET Frameworks. Learning these idioms will have you writing code like the native C# speakers in no time.
The second edition tracks the evolution of .NET and they apply as well to Mono. For instance, it now contains LINQ design patterns, extension methods patterns and DependencyProperties (used in WPF and Silverlight).