I have read the book. It does not teach you C. It assumes that you know the language. It does not teach you anything about the syntax of the language.

He does not tell you how to log into a UNIX system. He tells you what happens when you log into a UNIX system. (That is a big difference. Compare the first chapter of Kerrigan and Pike's The Unix Programming Environment, which actually does teach about terminals and explicitly talks about how to log in and what a login is -- for a complete beginner.)

Here's the first bit of code, from page 5:

    #include "apue.h"
    #include <dirent.h>
    
    int
    main(int argc, char *argv[])
    {
    	DIR		*dp;
    	struct dirent	*dirp;
    
    	if (argc != 2)
    		err_quit("usage: ls directory_name");
    
    	if ((dp = opendir(argv[1])) == NULL)
    		err_sys("can't open %s", argv[1]);
    	while ((dirp = readdir(dp)) != NULL)
    		printf("%s\n", dirp->d_name);
    
    	closedir(dp);
    	exit(0);
    }

My point is very simple: If you do not know C and especially if you also do not know any programming language, it would require either magic or another book or tutorial to figure out the syntax or semantics of those lines. Why is the first include thingy in quotes and the second in brackets? What is an include thingy anyhow? What is DIR? Why do some things have a * in front of them? When are semicolons used? Not every line ends in one, but most do - why? What is the -> used for? When do I use parentheses and when do I use braces? What is argc or argv? Are those names important? What is NULL? Why are some things all capitals and some not. Does that matter? He doesn't answer any of these questions (nor should he), because the book is not meant to teach you C. It assumes that you already know C.

It is not a beginner's book.

Hmm... Having not seen the first edition of the book (which is the edition you must have given that you used it 15 years ago and it was updated 8 years ago), it's possible that your assessment of its elementary content is specific to that edition.

However, the second edition of the text is by no means elementary; unless you are conflating beginner literature with comprehensive literature, and the text is certainly comprehensive.

I'll grant you that Chapter 1 is a summary and introduction to the entire book with little challenging or novel material for someone experienced with UNIX and C, but many advanced texts have introductory sections. A large portion of the "The Art of Computer Programming: Fundamental Algorithms" is essentially a review of the elementary mathematics necessary for the analysis of algorithms, but I would never call TAOP a "beginners book"!