I did not gain anymore insight from this book than I already had from reading MAN pages and briefly researching on-line. The first 25 pages are basically history, file structure and an introduction with tips on how to compile from source. The other 75 pages are shallow descriptions of various modules, dumbed down examples so things don't get to complicated, and a short section that talks about development. There is absolutely no depth and very little useful information. I can't believe this is called a "definitive guide". If your work as a system administrator has brought you to considering this book chances are you probably already know more about PAM than you will gain from this book. I gave it one star because it is thin, lightweight, and it uses nice big fonts.