. . The source code of the book in TeX (and a needed set of macros ) is available online on CTAN. It is provided only as an example and its use to prepare a book like The TeXbook is not allowed. . The full source code of TeX; also available on CTAN. Being written using literate programming, it contains plenty of human-readable documentation. . . contains extensive documentation about the algorithms used in TeX. . . . on AMS-TeX. .
  the source code of the book in tex (and a needed set of macros ) is available online on ctan it is provided only as an example and its use to prepare a book like the texbook is not allowed  the full source code of tex; also available on ctan being written using literate programming it contains plenty of human-readable documentation   contains extensive documentation about the algorithms used in tex    on ams-tex 