Upgrading Your Existing RPG Skills
RPG IV Jump Start, Fourth Edition
By Bryan Meyers
If you're already familiar with the basics of the RPG language, you don't need to start from scratch. This book teaches you the differences between RPG III and RPG IV, allowing you to update your skills.
Who Knew You Could Do That with RPG IV? A Sorcerer's Guide to System Access and More.
This is easily the best Redbook that IBM has ever published. It's focus is to teach someone who's already familiar with RPG IV, a series of advanced topics, including ILE, Subprocedures, APIs, Sockets, IFS, Database Access and so much more. The link is to a Partner400.com web page where they will explain how to search out the Redbook on IBM's site.
The Learning Center
Any time you want to learn a new System i related skill, there's no place better to shop that the Learning Center of the System iNetwork. There are books on every System i topic that you can imagine, plus there are e-Learning classes, Computer Based Training, Power Tips and more.