As a totally new user of Nagios, I found the book very useful in learning how to get started and setting up Nagios, although not complete.
It does cover loads of aspects and later chapters explain on how to expand from simple to advanced system checks and writing your own plug-ins, however it feels like the book was published in a rush, as several places placeholders of X exists, like covering a subject, see page X for more details. Sometimes sentences required a second glance because of odd word structures too.
At the end of the day however, if you can look past these minor issues, the book is absolutely recommendable if you like to have either a reference in paper or just are starting out with Nagios; google is great for examples of how to implement the things the book doesn't cover, it does point you in the right direction however ;)