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The TeXbook
(ctan.org)
A place to share ideas, resources, tips, and hacks for Donald Knuths typesetting software TeX. All variants and formats like OpTeX, LaTeX and ConTeXt are welcome.
True, maybe it would be better to have step by step instructions on how to compile your own TeXbook, since the source is free?
Edit: it seems like it's actually not 100% free to produce a pdf from the source.