Books! Things to know: It's a series. It was never finished though. As far as I know Frank wanted to write seven books but he only wrote six before he died. They go together, the first two do and then the next two. You should probably stop there, I did. If you do decide to keep reading then you should know that Brian (his son) wrote more but his work is not considered to be very good.
My opinion: The books are incredibly good! Dense, difficult to understand but if you're smart enough to then you will learn a lot. That's kind of the trick though, realizing that this is not an action story (like the movies are) but a story about humans. Frank is a fucking genius! All this shit that the system does, he saw it, how we function as species, he saw it. He put it into words, disguised it as a political story but as I said, once you realize what you're actually reading it's easier to understand. Of course you also need to be open minded as we are. Most humans are too dumb to understand any of it, that's why they see it as an action movie.