David Both 
All three volumes in the 2nd Edition of my Using and Administering Linux – Zero to SysAdmin series Linux self-study course have made the Amazon best-sellers lists in two categories. Of course these rankings are constantly changing but I was lucky enough to catch the lists at the right time to see this.
Linux Networking & System Administration
In this category Volume 2 ranks as #46, Volume 3 is #47, and Volume 1 is #48.
Linux Programming
In this category Volume 2 ranks in the #8 position, Volume 3 is #9, and Volume 1 is #11.
Jim Hall