Out of curiosity, why are you choosing the AS400 platform? I am over the AS400, Unix and HP Nonstop teams at my company. Is this for your current job?
This site provides more information than you could possibly need to know about i5 administration with V5.4 of i5/OS. With a little searching, I am sure you could find the equivalent site for another version of i5/OS.
The following site offers lots of free and chargeable training resources.
I hope this helps. Good luck.