Linux in a Nutshell (In a Nutshell (O'Reilly))
Ellen Siever
Aaron Weber
Stephen Figgins
Robert Love
Arnold Robbins

£34.15

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I've been using Linux for 10 years and I still periodilcally re-visit this book.

It's one of the tech books I've read cover to cover (a couple of times) and still each time I read it I've learned something new in the first 5 minutes.

The more I learn, the more concepts appear on the horizon, and so new parts of the book become relavent. 

Linux is complex and powerful, it has more features than you'll ever discover by accident; reading an alphabetical list of commands is a great way to find those undiscovered gems.

There's not much in this book that you won't find in man pages or online:  you don't need it as a reference. It's not a book for newbies - but if you're comfortable on the command line and want to know more it's a great book to read on the train.