Last week I read two books on Nagios. I found one easier to use than the other.
Nagios System and Network Monitoring (Wolfgang Barth, No Starch Press, 2006) is a delight. It explained Nagios briefly and clearly, showed me how to get it running, and continues to be a useful reference. Pro Nagios 2.0 (James Turnbull, Apress, 2006) has a lot of useful information as well, but is harder to read and understand.