There's an Ubuntu server guide dedicated to walking administrators through the Linux operating system, from installation to troubleshooting and from configuration to monitoring.
The Official Ubuntu Server Book, 3rd edition, by Kyle Rankin and Benjamin Mako Hill, acknowledges that not every admin will come into the Ubuntu Linux OS with a high degree of comfort, so it includes a section for learning the Ubuntu essentials. It also walks through the installation process. For those with lots of Ubuntu servers to keep track of -- or administrators who have other responsibilities -- there are even chapters on automation and monitoring.
What about security? The myth that Linux servers are immune to security threats is just that -- a myth. Chapter 6 of this Ubuntu server guide on security is all about using AppArmor, Secure Shell and key-based authentication as well as how to deal with common attacks such as botnets and other brute-force methods. From the security chapter's description of AppArmor:
... AppArmor is based on the security principle of least privilege; that is, it attempts to restrict programs to the minimal set of permissions they need to function. It works through a series of rules assigned to particular programs. These rules define, for instance, which files or directories a program is allowed to read and write to or only read from. When an application that is being managed by AppArmor violates these access controls, AppArmor steps in and prevents it and logs the event. A number of services include AppArmor profiles that are enforced by default, and more are being added in each Ubuntu release. ...
In addition to descriptions of security measures, Rankin and Hill present ways how to configure the tools to best protect your Ubuntu systems.
Editor's note: This excerpt is from the new 3rd edition of The Official Ubuntu Server Book, authored by Kyle Rankin and Benjamin Mako Hill, published by Pearson/Prentice Hall Professional, July 2013, ISBN 0-13-301753-2, Copyright © 2014 Canonical Ltd.