Assembly in LInux

Dear friends,

With all of your help and my motivation, I am to do application
programming in Linux using C.
I made some local intranet applications for my company using CURL,
MySQL, Pipe, and gnuplot.

But now I want to do some system programming under Linux. For that
purpose I need to study
Assembly Language.

I am studying two books to make programs using Assembly Language:

1. Assembly Language Step-by-Step: Programming with DOS and Linux,
Second Edition
2. Art of Assembly

I need your help to understand Assembly Language for Linux.
If you guys have some links or documentation.
Then please share with me. I am great thankful to you.

Thanks and Regards
Jai