This could easily be my general density speaking, but I don't understand the reasoning behind that choice of name for book 5. There were no masks in play in the book. There was death, sure, but that's hardly a distinguishing feature in this series. Quintus Cassius had a "mask" in the sense of being disguised, but that's only one mask, not multiple.
The closest thing I can come up with is that the Denarians themselves are metaphorical death masks for the people they used to be. It still feels wrong, though. What am I missing? Is there a blatantly obvious pun or connection here I'm just not getting?