Can you recommend a book about the AS/400 that is suitable for beginners?
I teach AS/400 classes in Ohio, and I have a few books that may be of interest to you. I will list them by category and you can then begin your quest...
AS400/OS400: Introduction to AS400 System Operations by Heidi Rothenbuehler & Patrice Gapen
This book will give you a good idea on how the hardware interfaces with OS/400 and the different ways AS/400s operate and perform.
Control Language (CL): Control Language Programming For the AS400 by Bryan Meyers & Dan Riehl.
I have also used this book for teaching. After 10 years of programming, I carry this book and refer to it from time to time.
Programming in RPG by Judy Yaeger.
I have used this book in the classroom, and I like it a lot.
All of these books are online at http://www.as400network.com/store/.
As I look back over the books I listed above, I would suggest reading them in the order they are listed. There are several sites, newsgroups, and people who can help you on your quest to understand this system. It is like nothing else in the industry and when understood and correctly used, it will outperform any system on the planet. Of course I am bias, but it will do whatever you want it to.