Most of it is pretty sparse--installing is easy, getting nagios working is harder. There is someone who used to be on the documentation list, Max Hetrick, who wrote some great nagios pages. Fortunately, much of his older stuff is still on the CentOS wiki. I would start there as far as Nagios.
http://wiki.centos.org/HowTos/Nagios
-- Scott Robbins PGP keyID EB3467D6 ( 1B48 077D 66F6 9DB0 FDC2 A409 FA54 EB34 67D6 ) gpg --keyserver pgp.mit.edu --recv-keys EB3467D6
I have done this not so long ago and I am using OMD distro, they call it a distro but it's really not. Here is the site with additional information - http://omdistro.org/start
They have lots of stuff rolled up and ready to go, makes it fast a quick to get something up and running, using check_mk pretty cool.
I have it loaded on a centos 6.5 box, here is the download link - http://omdistro.org/start but about halfway down the page they have a link to the package repo's, add the repo and install, will need to have elrepo in there for dependencies. Hope this helps.
Highly recommended =)