GCC Warning: implicit declaration of function 'puts' is invalid in C99

This "book" should be renamed to Learn To Hate C By Following Meaningless Examples That Are Blatantly Wrong.

The correct code in modern C would be plain and easy

#include <stdio.h>        // include the correct header

int main(void) {          // no need to repeat the argument mantra as they're not used
    puts("Hello world."); 
}                         // omit the return in main as it defaults to 0 anyway

While the original example

int main(int argc, char *argv[]) {
    puts("Hello world."); 
    return 0;
}

would have been just plain bad in 1989, in 1999 (that is 18 years prior to the writing of this answer, and almost as many years before the "book" was written) the C standard was revised. In the C99 revision, this kind of implicit function declaration was made illegal - and naturally it remains illegal in the current revision of the standard (C11). Thus using puts without #includeing the relevant header, i.e. prepending #include <stdio.h> (or declaring the puts function with int puts(const char*);) is a constraint error.

A constraint error is an error that must cause the compiler output a diagnostics message. Additionally such a program is considered an invalid program. However the peculiar thing about the C standard is that it allows a C compiler to also successfully compile an invalid program, though a compiler may as well reject it. Therefore, such an example is hardly a good starting point in a book that is supposed to teach C to beginners.