There are two main popular Linux families, Red Hat and Debian. They're cousins, but they do things very differently. Their children act like their parents. Ubuntu is a child of Debian. Fedora, Mandriva, and SuSE are children of Red Hat. Red Hat and Debian have some other cousins too, like Gentoo, Slackware, etc, but their families are a bit smaller.

If you are buying a book to help you learn, make sure it goes with the distro you choose. Most Linux system administration books are aimed at Red Hat system administration. They will not always work for Ubuntu. You need a Debian book if you intend to practice on Ubuntu. Debian books won't work for Red Hat systems either.

I got a Mandriva book when I started. It explained KDE and Red Hat things. The only use I ever had for it was to get the octal values for chmod. Don't waste your money like that. And if you're getting frustrated because Ubuntu doesn't match the book,'re using a Spanish dictionary to translate German.