If you want to become a professional and not just a dabbler I would recommend reading some of the following books I have in my bookshelf:

[0] RHCSA & RHCE Training and Exam Preparation Guide by Asghar Ghori. This book will help insure you know your stuff as your system engineer/administrator wise.

[1] A Practical Guide to Linux Commands, Editor and Shell Programming Third Edition. This book will cover the majority of what you would need and want to know when connecting to a remote linux system over ssh.

If you want to get under the hood and become an expert, the following books should help get you started:

[2] Advanced Programming in the UNIX Environment

[3] The Linux Programming Interface: A Linux and UNIX System Programming Handbook

[4] Linux Kernel Development 3rd Edition

To get a nice general overview and get up and going quickly:

[5] How Linux works: What every superuser should know

[6] The Linux Command Line

[7] Python Crash Course

[8] Automate the boring stuff with Python. This is a great book to help you think about how to automate most of the repetitive things you will end up doing on a regular basis.

[0] https://www.amazon.com/RHCSA-RHCE-Red-Enterprise-Linux/dp/14...

[1] https://www.amazon.com/Practical-Guide-Commands-Editors-Prog...

[2] https://www.amazon.com/Advanced-Programming-UNIX-Environment...

[3] https://www.amazon.com/Linux-Programming-Interface-System-Ha...

[4] https://www.amazon.com/Linux-Kernel-Development-Robert-Love/...

[5] https://www.amazon.com/How-Linux-Works-Superuser-Should/dp/1...

[6] https://www.amazon.com/Linux-Command-Line-Complete-Introduct...

[7] https://www.amazon.com/Python-Crash-Course-Hands-Project-Bas...

[8] https://www.amazon.com/Automate-Boring-Stuff-Python-Programm...

There is also a 3rd Edition which was released in 2012.

https://www.amazon.com/Practical-Guide-Commands-Editors-Prog...

I like this:

http://www.amazon.com/Practical-Guide-Commands-Editors-Progr...

I've also found "Unix for the Impatient" very helpful but the 1995 copyright is a little scary.