If, like me, you found Nagios to be rather confusing to configure this is definately the book for you. It goes into great detail on installing the software from source, setting up the software and using the interface. It also details agents that can be installed on Linux and Windows hosts that monitor the processes, disc usage and network interfaces. If you are a lazy/smart admin this is the book for you: just set it up and it will tell you what wrong on your network and servers and save you a load of time. Sure, it'll take a while to set up and tune but the benefits of having it once you get it right will leave you thinking how you managed without it.
It includes "parents" on the network to quickly determine what is at fault; so where you get a user bleating "I can't access the Internet" rather than doing your fault finding (check the proxy, check the firewall, check the router, phone the ISP) Nagios will tell you where the fault is before the user even picks up the phone.
It even talks about monitoring the room temperature of the server room and letting you know if there is a fault there.
Many technical manuals don't provide a decent index; this is not one of them! The index is great and on level par with "Unix Power Tools" by O'Reilly.
Linux experience is required to some degree, but don't let that put you off. Most linux distributions are easy to install now and get up and running to run Nagios. If you're not sure what distro to use (and haven't bought a RedHat pre-installed server) then Ubuntu Server is a great place to start and has excellent device driver support.
I don't normally write reviews but this is by far the best technical manaual I have ever read (and I've read a lot) so felt compelled to shout about it from the rooftops - if you're looking to learn Nagios look no further.
If I had one complaint, it is that it does not cover NConf - read this book and then check it out!