Honestly it seems like an arbitrary line, considering that Herbert's story continues for a few more books after that. The first two books could also be considered a good standalone (duology?), since they cover the span of Paul's story.
Really, the only wrong answer is the first book alone. I think most people read only the first book and get the wrong idea.
There's a couple of private trackers for STLs:
materialize.is (main)
cgpeers.to (to a lesser extent)