There are a number of resources, both online and offline, where you can more information about user authentication. If you know of any resources that ought to be added to this list, drop me a line at <>
Linux-PAM System Administrator's Guide
Linux-PAM Module Writer's Manual
Linux-PAM Application Developer's Manual
A lot of information can be gathered from your system's manual pages. The following are some manpages relating to user authentication. The number in parentheses refers to the manpage section. To view the passwd(5) manpage, you would enter man 5 passwd.