Patterns are common reaccurances in OO design. Despite this somewhat simplistic description of patterns, its not entirely what the book is about (although it may seem at first glance - read the description on Amazon.com or browse the book at your book store). I consider this book somewhat similar to some of the great Philosophy books that Ive read; each sentence has so much wealth of knowledge in it, youll find yourself re-reading them several times.
I should warn you, the book isnt for the feignt of heart. Make sure you are very familiar with OOP before reading it. When I first starting reading this book, I was like "what?" I revisited it 6 months later, and had that "aha!" .. as if a lightbult went off in my head. Not only has my programming and design greatly benefited, but it has also given me a far better understanding of the .NET framework and how it all works.