Books about (La)TeX
Tout ce que vous avez toujours voulu savoir sur LaTeX sans jamais oser le demander
Vincent Lozano's Tout… sur LaTeX is licenced under the licence art libre, typeset using LaTeX. An introduction book in French, clear and whose reading is rather easy.
TeX by Topic
Viktor Eijkhout's TeX by Topic is licensed under the GNU Free Documentation License (FDL). It is typeset using LaTeX, and the source is available at GitHub here.
The question asks specifically about LaTeX, but the one and only The TeXbook by D.E. Knuth himself is a fine example of a complete book typeset with TeX. Its source is available on CTAN as the package
texbook. Please note that this source is available for inspection/learning only and is not to be compiled.
LaTeX for Complete Novices as well as all other titles in The Dickimaw LaTeX Series
This collection of books written by Nicola Talbot is licensed under the GNU FDL just like TeX by Topic. The sources for each book1 along with the typeset books in an array of output formats are available at the respective home pages:
1 A caveat potentially relevant to this question is noted near the download link for each book's source code:
Note that the source code is very complicated as it required creating four different versions of the document: the HTML version, the A4 PDF version, the screen PDF version and the 7x10 inch paperback version. It should not be considered an example file for beginners.