Actually, I got that from Expert C Prog by Peter Van Der Linden (Sun OS compiler + kernel group).
Table A-1, Library calls v System calls.
As mentioned, Sect 3 of Unix man pages is lib calls eg system, fprintf, malloc, Sect 2 is system calls eg chdir, fork, write.
An extremely good and easy to read book.
Here's the quote:
"Finally, people who believe that crop circles are the work of aliens will have trouble with the concept that the system() call is actually a library call