Prodigy

Build of Suspense
Wow!  Plenty of sequels fall short or don't add much, but Prodigy by Marie Lu is not one of them.  It is one of the rare sequels that adds depth and distinction to the original plot, and pulls further into the character's innermost thoughts and gives you the opportunity to see their true character.  Allegiances lie in different places than originally expected and new characters are revealed to be untruthful and power-hungry, giving the original protagonists of June and Day new obstacles to overcome as their power together strengthens.
This strong build of suspense as they uncover the horrid truth behind a plot to kill the new Elector and must run to save his life and rescue the country from the true dictator is heightened as June becomes sick on the journey and must spend time in the hospital.  This chips away at the precious few days they have to rescue the Elector and get the word out before the man who plots to kill him and take over the Republic does so.
The build was very effective throughout because of this and forced me to keep reading the book for hours until I finally read the last word.  Even the ending is a cliff hanger and sets the stage well for the soon-to-come final book in the trilogy, Champion.