I would also recommend "R. G. Loeliger Threaded Interpretive Languages Their Design And Implementation" between these two books the whole beauty of Forth and their implementation should just click.
Forth isn't one of those languages that you _use_. You extend the language from the inside, so you need to know how your Forth is implemented. I'd say it is the only language where users of the language could all recreate the language.
Now I'd love the same with Starting Forth set to ANS Forth standards, and not just in web form. Yes, I know how to use wget --mirror and such, but I'm used to MuPDF and the editor terminal switching back and forth. No pun intended.
Forth isn't one of those languages that you _use_. You extend the language from the inside, so you need to know how your Forth is implemented. I'd say it is the only language where users of the language could all recreate the language.