Not exactly a textbook, but of the Linux books I have, "Linux System 
Administration, A User's Guide" by Marcel Gagne (Addison Wesley) is the most helpful 
for me.  It covers a lot of ground at beginner to intermediate level, with not 
too much or too little detail, in my opinion.  I also refer to my copies of 
"Linux Administration Handbook" by Nemeth, Snyder, and Hein (Prentice Hall), 
which incidentally has exercises like a textbook, but is a little too deep in 
places, and "Running Linux" by Matt Welsh (O'Reilly), which I think is good for 
"generic" information but harder to relate to questions about specific 
distributions of Linux.

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