I don't get why you would need a book to learn how to use ubuntu. Linux, yes, but not ubuntu specifically.
Generally anything from here: http://oreilly.com/linux/ will be informative, not to mention factual. O'Reilly books are excellent, if a little expensive.
There are, of course, differences among different Linux distributions/versions. For example, for package management Debian-based Ubuntu uses apt-get, Red Hat-based distributions such, as Fedora, use yum, etc.. Most likely this isn't a big deal, though.
I was fond of Fedora Unleashed, so I would expect Ubuntu Unleashed to be good, as well.