Most of the books about specific GNU/Linux distributions that I've read tend to be either too advanced to comprehend or too elementary to stomach. Apress's Beginning Ubuntu Linux tends toward the latter, but doesn't totally ignore more advanced GNU/Linux topics.
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Beginning Ubuntu Linux is the book that it claims to be, though it's not quite as good as it could be. In general, “beginning Linux” texts tend to suffer from the same oversimplification found in this book. It seems that no one has hit this genre squarely yet. Still, it's as good as it gets right now, and I would recommend Beginning Ubuntu Linux over any other introductory GNU/Linux book — at least, as of this writing.