The Ubuntu Pocket Guide and Reference (written by Keir Thomas) is an easy to understand reference guide to Ubuntu, the free Linux operating system. In seven chapters it explains the essentials of day-to-day Ubuntu administration in a very simple manner. Installing Ubuntu, with in Windows, on a dual booting system, on a virtual machine, on an Apple Mac, installing drivers, software, sound, Bluetooth, installing printers, scanners, removable media, etc. etc are very neatly described in this book.
The PDF Edition of Ubuntu Pocket Guide and Reference is available entirely free of charge. It is practically identical to the Print Edition. You can download the free e-book from here.