I am a professional programmer, and having read both this and the Gang of Four design patterns book, I can honestly say I prefer this one. The GoF book is excellent if you need to discuss potential options for software design with colleagues, and pretty good if you need to implement one of them. However, the patterns in this book (POSA) are more wide-ranging, useful, and practical than those in Design Patterns, which in comparison appear too theoretical.
I would suggest that reading Design Patterns first is a good introduction to the field of patterns. After that, read this book to see the power of patterns. The explanations of layers (eg ISO/OSI), broker systems (eg CORBA), and MVC systems (eg most spreadsheets) as generic systems are absolutely second to none.