How to learn UNIX/Linux

There are a lot of books... however, I recommend reading the manuals.
I know... it's a lot... but, I did it (3 decades ago) and it works really well.

Here are a few sites with unix manpages:

http://www.ma.utexas.edu/cgi-bin/man-cgi 
http://unixhelp.ed.ac.uk
http://bama.ua.edu/cgi-bin/man-cgi

I'm sure there are other good sites with manpages as well.

I recommend you read the entire manual, end-to-end - but unless you want to do that twice or thrice you'll want to skip around, and learn these commands first.

-> learn bash really well.  Read every single line of the manual, and figure out what the heck they're talking about.  Learn every variation of how to tweak a variable, how to do filename expansion (globbing, as opposed to regular expressions), what all the builtin functions are and how they work.

-> learn vi (or vim, if on Linux) - exceedingly awesome editor.  Sure, use a tutorial, and don't bother to dive down the ratholes of macros or settings tweaking unless you're really into those things, 'cause you'll wake up months later wondering what happened.  If you're in a hurry, learn nano instead.  You won't be as happy, but you'll be able to edit files.

-> learn awk.  Awks associative arrays can help you solve some serious problems.  Once you know how to use awk, you'll never look at a flat file or a "unix pipeline" quite the same.  You can't learn too much awk.

-> learn sed.  Also awesome, both for modifying files on disk and for applying mods to every line of text fed through a "pipeline" to it.

With those four as "cornerstones", you'll be a UNIX power-user in no time.  They don't all work the same, or have the same conventions... but you'll get over the hurdles and be all the better UNIX expert in the end.

ALSO - once logged in - the 'man' and 'apropos' commands are your friends.  Read, learn, experiment, and build up your commandline/pipelines piece by piece.  Start small, and build.

Example of "building up" a command line:

find . -type f -print
...show me all the files in the current directory and down

find . -type f -print0 | xargs file
...tell me what kind of file each of them is (and, handle filenames with spaces in them properly)

find . -type f -print0 | xargs file | egrep -v GIF\|JPEG\|PNG\|PDF
...show me just the files that are NOT gifs, jpegs, pngs, or pdfs.

UNIX commands are kind of like LEGO elements - you can plug them together in cool ways.  So, get used to building up pipelines that do what you want.

I guess, most of all... don't expect to be given a guided tour.  

Instead, treat UNIX like a huge machine, where each part of the machine does have a manual and can be understood, and make it a habit, every day, to spend time wandering around in that machine, exploring it end-to-end, corner-to-corner, until you know it well enough to call yourself a UNIX power-user.