This isn't an ebook, but an online guide, but it's one of the easiest to understand, if what you want is to learn Ubuntu linux.   http://www.psychocats.net/ubuntu/index.php   If you want to access it offline, I guess you can save it to your hard drive after getting permission from the author(you can post a comment in his blog http://ubuntucat.wordpress.com/about/   or in this ubuntuforums thread http://www.psychocats.net/ubuntu/index.php )Hope you find it useful. It's one of the sites I always recommend to anyone who's interested in switching to Linux and wants to find out more. You can also find more info on other Ubuntu resources in the guide.All the best.