Keir Thomas the author of the book Ubuntu Pocket Guide and Reference was kind enough to release the full version of the book for free in the form of a PDF. The focus of the book is on explaining Ubuntu in detail, the various chapters cover the desktop environment and the command line. Give the book a try and read for your self. Remember to support Keir Thomas effort and time by buying the print edition.
You can buy the print edition from Amazon.com for $9.94.
Download the PDF edition of the entire book for free.