"linux in a nutshell"---O'Reilly
Also, a really good resource is TLDP (The Linux Documentation Project). One thing to get there is "Bash Guide for Beginners" by Machtelt Garrels.
BUT--Don't read too much before actually doing---I find that the best way to learn is to "push buttons". When I can't figure out how to do something, then I go looking for it in the book.