I wholeheartedly agree; great book. I, too, bought from Amazon but I bought the book before I knew it was on-line.

However, as Steve BZ notes, the on-line version is not a good substitute for the hardcopy when you're just reading it. I was glad I had the hardcopy.

But the on-line version is a *great* reference in my opinion. I use it a lot as I go back and rework and rework and review and try to digest all of the nuances in that book.

I have been periodically going back to that book every several months to review and better understand different parts of it. I find the on-line version very useful and accesible in those short but deep dives into a piece of the work.

Of course, YMMV.