It's just the basis for all the other stuff on 43 Folders. 43 Folders is an implementation details site; the book is the philosophy and reference guide. None of the system tweaks really make sense without the book.
As to the copious linkage, it may be that the kind of people who take to GTD are the kind of people who like things to be "correct" (like, say, engineers) and thus must link every book mention to the Amazon page, because that's how linking works, dammit. I dunno.
You're reading the advanced tips guide and wondering why it keeps pointing back at the main guide and reference. That's the only way it all makes sense.
(In short, Getting Things Done is so great that everyone wants to push the book.)
The Book
As to the copious linkage, it may be that the kind of people who take to GTD are the kind of people who like things to be "correct" (like, say, engineers) and thus must link every book mention to the Amazon page, because that's how linking works, dammit. I dunno.
You're reading the advanced tips guide and wondering why it keeps pointing back at the main guide and reference. That's the only way it all makes sense.
(In short, Getting Things Done is so great that everyone wants to push the book.)