Some very good observations.

Otherwise a few points, from the copy here (clearly a 1st ed): http://cs.upm.ro/_users/cursuri_on_line/Alte_documentatii/C%...

"void didn’t exist in the book (although it did exist at some point before C89; see here, section 2.1)" -> There are many occurrences of void in the document above.

"It appears that stdio.h was the only header that existed" -> From the document above many other headers are available (string.h, math.h, etc.)

"Note that return 0; was not necessary." (on the main function) -> return 0 is not necessary in Standard as well (even if the semantic of omitting it is different between C89 and C99).