Donald Knuths surprisingly friendly manual of the TeX macro language.
Edit: Removed the link to the pdf, which is available online in various places, because of copyright. The source is freely available to read, but not to make your own document.
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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.