I don't know if its NDA'd, but if not, see if you can find out the sort of qns they will ask/tasks they will require.
Also, do they focus on a particular distro, or is it neutral?
There are a lot of Admin manuals, inc RHEL/Centos v6 here http://www.linuxtopia.org/online_boo...ion_index.html
I would imagine the Essentials & Deployment guides should cover it pretty well.
This is worth having as well http://rute.2038bug.com/index.html.gz