Dude, I work on the AS/400 (now called iSeries) and it's operating system, i5/OS , that it employs every day for work. It's part of my job.
I whole heartedly recommend this book,
IBM i5/iSeries Primer, Fourth Edition , for anyone who wants to become familiar with the machine and it's intricacies for a start.
Is there a specific question you need answered?