I personally enjoy the "netinstall" method of downloading just what you need. Here is some random guy's tutorial, looks accurate to me:
On the other hand, the DVD has all the essential packages so it's handy for computers that aren't connected to the Web or for multiple installs. In fact I recommend doing lots of "practice" installs. For example create several virtual machines and do a server install, desktop install, terminal install, etc. and get them all talking to each other over the virtual network.
As for essential reading materials, start here: https://www.centos.org/modules/newbb...m.php?forum=54
If you are looking for printed material the Michael Jang RHCE book is highly-rated.
Also a good idea to familiarize yourself with CentOS 5.x and even 4.x as you will certainly encounter these on the job.