Reddit reviews Linux Command Line and Shell Scripting Bible

Knowing C++ in the field of Linux administration might help you when you need to debug or compile a library or application that is written in C++, and sometime you need to.

knowing C++ well can benefit you programming skills a lot, as lots of important software in the world written in this language.

Here's a good book.
http://www.amazon.com/Advanced-Linux-Programming-CodeSourcery/dp/0735710430

If you wish to really improve you Linux administration skills i would go for BASH:
http://www.amazon.com/Linux-Command-Shell-Scripting-Bible/dp/047025128X

Oh. In that case I'm not sure what the definitive ones are but any of *these should give you a good idea.

• http://www.amazon.com/Beginning-Linux-Command-Experts-Source/dp/1430218894/ref=zg_bs_3849_89 or http://www.amazon.com/Linux-Command-Shell-Scripting-Bible/dp/047025128X