Of the books that deal specifically with Nagios, this is a good one. It goes into greater depth on topics the product documentation doesn't, such as passive checks. On other topics it makes a good companion to the documentation, providing additional examples and alternate explanations.
Only down sides are several typos, and that Nagios v3.0 is nearly ready for release! So I can only hope these authors are encouraged enough to do a second edition when that happens.
