I think this is really an indispensable book
for any unix system administrator.It gives a lot
of well structured and useful information for
the main flavors of unix, but what I really loves
is that it EXPLAINS the basis, the essentials.

It has been a great help on my job, and I've
been surprised to read the advices and comments
about system administrator virtues, and to check
they coincides a lot with my own thoughts on the
matter.
It is one of the best O'Reilly books I've purchased.