I'm a Ruby programmer and need to learn RPG IV because of my current project. I want to learn primitive types, data structures, control flows, architecture etc. Online resources are as good as paper books.
What book or online tutorial should I start with?
This Red Book from IBM is still quite relevant today.
IBM i Information Center is the source for the actual documentation.
RPG-L is a great mailing list with hundreds of helpful people to answer all of your questions.
Another link I knew was out there, but couldn't find at first was from Scott Klement who is well known in the IBM i community.