The book is essentially a survey of various Linux system-administration tasks: installing Debian; setting up LAMP; configuring a load-balancing, high-availability environment; working with virtualisation. None of the chapters are in-depth examinations of their subjects; rather, they're enough to get you started and familiar with the concepts involved, and headed in the right direction. I like this approach, as it increases the likelihood that any particular admin will be able to use the material presented. I've been working with Apache for almost a decade now, but I've not done any virtualisation; some other fellow may have played with Linux for supercomputing, but never done any web serving with it; we both can use the chapters which cover subjects new to us.
I really like some of the choices the authors made. A lot of GNU/Linux 'administration' books focus on GUI tools — I've seen some which don't even bother addressing the command line! I've long said that if one isn't intimately familiar with the shell — if one cannot get one's job done with it — then one isn't really a sysadmin. Linux System Administration approaches nearly everything from the CLI, right from the get-go. Kudos!
The authors also deserve praise for showing, early on, how to replace Sendmail with Postfix. In 2007, there's very, very little reason to use Sendmail: unless you know why you need it, you almost certainly don't. Postfix is more stable and far more secure.
Another nice thing is how many alternatives are showcased: Xen & VMware; Debian, Fedora & Xandros; CIFS/SMB & NFS; shell, Perl, PHP & Python and so forth. One really great advantage of Unix in general and GNU/Linux in particular is choice — it's good to see a reference work which implicitly acknowledges that.
The authors are also pretty good about calling out common pitfalls — several got me, once upon a time. It'd have been nice to have had a book like this when I was cutting my teeth...
Lastly, I liked that the authors & their editor weren't afraid to refer readers to books from other publishers, in addition to O'Reilly's (uniformly excellent) offerings. Not all publishers would be so forthright; O'Reilly merits recognition for their openness.
The book's not quite perfect, though. I wish that PostgreSQL had at least been mentioned as a more powerful, more stable (and often faster in practice) alternative to MySQL, and one doesn't actually need to register a domain in order to set up static IP addressing. Still, these are pretty minor quibbles.
I'd say that the ideal audience for this book is a small-to-medium business admin who'd like to start using Linux, or who already is but doesn't really feel confident yet. It covers enough categories that at least a few are likely to be relevant. Even an experienced admin will probably find some useful stuff in here."