Linux System Administrators' Guide describes the system administration aspects of using Linux. It is intended for people who know next to nothing about system administration (as in "what is it?"), but who have already mastered at least the basics of normal usage.
System administration is all the things that one has to do to keep a computer system in a useable shape. It includes things like backing up files (and restoring them if necessary), installing new programs, creating accounts for users (and deleting them when no longer needed), making certain that the file system is not corrupted, and so on. If a computer were, say, a house, system administration would be called maintenance, and would include cleaning, fixing broken windows, and other such things. System administration is not called maintenance, because that would be too simple.
The structure of this manual is such that many of the chapters can be usable independently. This makes the book easier to use as a reference manual, and makes it possible to read just a small part when needed, instead of having to read everything. However, this manual is first and foremost a tutorial.