Here's the Ubuntu Desktop Guide, a comprehensive guide to using Ubuntu in a desktop environment.
The guide is divided into the following chapters:
Getting Started – An introduction to the world of Ubuntu and some basic concepts which are useful when getting started.
Adding, Removing and Updating Applications – How to manage your software in Ubuntu.
Common Tasks – Performing common tasks on Ubuntu, including playing music and video, using the internet, office and graphics programs and more.
Configuring Your System – Change the settings of your Ubuntu system according to your needs.
Getting More Help – Where to find help if this guide doesn't provide the answer.