While the rest of this series gets a totally readable "2," The Kill Order ups the difficulty game a tad.
See, The Kill Order is a teensy bit hard to read in the beginning… because we're used to reading about the Maze. And the Gladers. And Thomas, Minho, and Teresa. So it's a little tough to shift thirteen years back into a past that we only get mere glimpses of during the first three books.
Nevertheless, we get used to it. We figure out who the main characters are and what their personalities are like. Then stuff starts clicking. People dying of poison? No problem: that's the Flare. People shooting darts at innocent civilians? No problem: that has something to do with the government.
We know a lot about the past because we've read the first three books, but now we actually get to visualize it. So once you get past the whole, "Where are the bloody Shanks?" problem, you'll be all set to go with The Kill Order.