(3) Base Camp
Just as the leap from Divergent to Insurgent didn't require pursuing a master's degree in Post-Apocalyptic Studies, Allegiant is at the same reading level as the two books before it. Tris ends up leaving Chicago, but everyone in the outside world talks the same as the people inside the fence. There's no new dialect, language, or slang to learn in order to understand what's going on.
In fact, everything is pretty straightforward. The serum that erases memory: memory serum. The serum that causes death: death serum. We heard there's a serum that causes people to speak in really long words: supercalifragilisticexpialidociouserum—but it's still in beta testing.