If you’re going to have long relations with Linux operating system and use it as efficiently as possible I suggest to read the following howtos and guides that will help you on the way to advanced Linux user status
Advanced Bash-Scripting Guide by Mendel Cooper
This document is both a tutorial and a reference on shell scripting with Bash. It assumes no previous knowledge of scripting or programming, but progresses rapidly toward an intermediate/advanced level of instruction. The exercises and heavily-commented examples invite active reader participation. Still, it is a work in progress. The intention is to add much supplementary material in future updates to this document, as it evolves into a comprehensive book that matches or surpasses any of the shell scripting manuals in print.
GNU/Linux Command-Line Tools Summary by Gareth Anderson
This guide was created as an overview of the Linux Operating System, geared toward new users as an exploration tour and getting started guide, with exercises at the end of each chapter. For more advanced trainees it can be a desktop reference, and a collection of the base knowledge needed to proceed with system and network administration. This book contains many real life examples derived from the author’s experience as a Linux system and network administrator, trainer and consultant. We hope these examples will help you to get a better understanding of the Linux system and that you feel encouraged to try out things on your own.
The Linux System Administrators’ Guide by Lars Wirzenius and others
This book assumes knowledge of everything in the Installation and Users’ Guides. It will cover all of the aspects of keeping the system running, handling user accounts, backups, configuration of the system, installing and upgrading software, and more. Whereas some of this information is in the Installation Guide (just to get the system off the ground) this book should be much more complete.
Linux Advanced Routing & Traffic Control HOWTO by Bert Hubert
Linux has very advanced Routing, filtering and traffic shaping options. This howto attempts to document how to configure and use these features.
Iptables Tutorial by Eric Steven Raymond
Iptables is an extremely knowledge intensive tool. This means that iptables takes quite a bit of knowledge to be able to use iptables to it’s full extent. Among other things, you must have a very good understanding of the TCP/IP protocol.
Linux Filesystem Hierarchy by Binh Nguyen
This document outlines the set of requirements and guidelines for file and directory placement under the Linux operating system and also its actual implementation on an arbitrary system.
Securing & Optimizing Linux: The Ultimate Solution by Gerhard Mourani
Mastering security with Linux and getting the maximum out of your system have never been easier. Securing & Optimizing Linux: The Ultimate Solution has been written and achieved with tightening security to an incomparable level in mind. One of its main features is the easy path from beginning to end in a smooth manner, step by step for beginners as well as for experts.