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Notice that while the linked page says "You can read the whole book, for free, online", chapters 18 to 30 haven't been written yet (e.g: http://craftinginterpreters.com/types-of-values.html).

Slightly below the button marked "start reading" there is a section entitled "What’s the Catch?" that explains as much.

I was about to write that very comment. It's not like he's hiding it, and he's giving the (e)book out for free, what more could you ask?

The same was true of the author's previous book, Game Programming Patterns, which is now complete and fully published.

(... and still free online!)