I have just finnished reading the excellent book "Learning Nagios 3.0" by Wojciech Kocjan.
The book does not require any prior knowledge about Nagios but having basic knowledge in programing, Unix or Linux is good. I as a rather experienced Nagios admin did get some valuable hints by reading it. I really liked the chapter about differences between Nagios 2 and 3.
The books target audience is sysadmins with Unix or LInux background that want to Monitor their environment by using Nagios. For a beginner going to the Nagios homepage the information is overvelming and it has to know where to start, a book like this is handy. The book often refers to specific topics at Nagios documentation for more info and that is good if the reader wants to dig deeper into that topic. Nagios homepage is an excellent reference but not a place to start when you want to use Nagios.
The examples are good and there is alot of illustrations, sometimes the illustrations is more complicated then the topic itself. The writer shows deep knowledge about the subject and has done a good job getting it technically correct, sometimeas a little bit academic i.e. it is more important to get everything correct then explaing something in a clear way.
I would recommend anybody interested in using Nagios to buy and read this book, especialy if you do not have any prior knowledge to Nagios.