Which book i can refer that is available on internet free of cost from which i can learn basic linux commands?

I have been using a Linux distribution Ubuntu for about one month and i think that i am not able take the full advantage of Linux's terminal power which differentiates it from windows. Just want to become little bit more productive on my linux system. If you can then please paste a link in addition to just referring to the name of such a book. My basic purpose is to learn the terminal . I would love to read and learn how to master ubuntu's terminal.