Among the Enemy - Margaret Peterson Haddix
Whew, it's been a while since I read a book from this series!

Matthias, Percy, and Alia, characters introduced in the third installment, Among the Betrayed are reintroduced in this novel. They are living at Niedler School when the Population Police take them to what seems to be a work camp. They escape and run to the woods, but Matthias ends up working "among the enemy," at the Population Police Headquarters as an officer-in-training, undercover. There, he finds Nina, and they try to come up with ways to fight the Population Police.

I understand the protagonist changes are to get a better understanding of this dystopian society, but I still wish the series stuck with Luke/Lee as the MC for the entire run. I'm not sure why it bothers me. Or why it bothered me when Lee's mention was "'Lee - remember Lee? - he shovels out the stall where the top officers have their horses.'"

But Matthias was a pretty good lead. A sweetie with relatable, conflicting optimistic and pessimistic thoughts.

I'll have to grab the last one soon. It will be nice to finish this series!