Try the embedded linux site at http://elinux.org/RPi_Hub
And what seems to be the most popular operating system http://www.raspbian.org/
Also the Debian Administrators Handbook - download free of charge from http://debian-handbook.info/
(Raspbian is built from Debian so uses the same commands).It was translated published after group fund-raising so if you can make a donation (to enable updates in the future) they would appreciate it. Or you could buy the book from Amazon.
Finally, to encompass just about everything you'll need to know about Linux try RUTE ('Rute Users Tutorial & Exposition) at http://rute.2038bug.com/index.html.gz
free of charge (but might be best to leave that for a week or two) 8)
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