It's rare for a sequel to surpass the original but I loved Sapphique even more than Incarceron. It is definitely necessary to read Incarceron first, nothing in Sapphique will make sense if you don't. Sapphique takes us back to the prison where Attia and Keiro are both feeling betrayed at having been left behind in the prison. They struggle to find an alternate way out while wrestling with their opposing feelings about Finn having left them behind.
Outside, the political intrigue in the Realm gets thicker as on the day when Finn is to be recognized as Prince Giles, the true heir of the Realm, another man steps forward to claim the title. There's a delicious back and forth through most of the story as the reader is pulled at once time towards believing Finn is indeed Giles only to be pushed a moment later towards believing he is not.
The storylines collide towards the end of the book as both worlds begin to crumble. The finale comes to a crashing conclusion and while I felt satisfied in the ending, there is some wiggle room for extending the series.