Who is Nona? No one, least of all Nona herself, actually knows who's soul resides in the body originally belonging to Harrowhark. Her memory begins six months ago on planet dominated by different rebel factions and threatened by one of the Resurrection Beasts. Her friends, Pyrrha Dve (in the still living body of the dead Lyctor Gideon the First) and Camilla Hect, (who is sharing her body with her dead necromancer Palamedes Sextus), have only told her that they believe her to be one of two friends. Their supposed allies from the rebel group Blood of Eden are only allowing her to live because they hope her soul belongs to Gideon Nav who they need to open the Locked Tomb. Nona's personality is cheerful and completely innocent with only occasional flashes of what might be either the irreverent Gideon or serious Harrow. As Nona struggles to make sense of the events around her while her friends analyze every bit of her dreams she can recall in the hopes they are the key to unlocking her soul.
There are three important continuing storylines in Nona. One is the first puzzle presented in the series, which is how someone becomes a Lyctor. The end of book two revealed that John, aka Necrolord Prime, aka God, had lied about the necromancer needing to destroy the soul of their cavalier. He didn't destroy his "cavalier" which leads to the second plotline of interest in Nona, being the identity of the one in the Locked Tomb, why John sealed her away, and why the rebels need Nona to be Gideon Nav. The largest of these storylines is related in Nona's dreams where she sees a man and woman although she doesn't understand the places they go or what they discuss. The dreams are John relating his story. Our Sun was dying and he was part of a research team developing cryostasis so humans could travel to other worlds when money and politics got in the way. Its an interesting and occasionally humorous story involving walking dead presidents, dead cows, nuclear weapons, cultists, evil trillionaires, science nerds, a nun, and at last a necromancer.
Sadly, those relevant and interesting details only account for thirty percent of the book. The rest is taken up by Nona working in a small classroom and making friends with a gang of kids. She's a nice, friendly, innocent girl who sees nothing unusual about the fact that she can't die. Its very well written, but it appears that Muir wrote Nona the Ninth as a bridge between Harrow the Ninth and Alecto the Ninth and had to fill out the pertinent parts to make it novel length. The story isn't quite as disjointed as Harrow the Ninth was with its alternate mixed up retelling of Gideon the Ninth but there is inadequate explanation given for how "Nona" ended up on this planet and why the leadership of the Sixth House was there. Since Harrowhark's body was last seen with Pyrrha I assume she had something to do with it but how they escaped the spirit world "River" and hooked up with Camilla/Palamedes is a mystery to me. Maybe I just missed it.
Still, now that we know how it really all began I'm looking forward even more to finding out how it all ends in Alecto the Ninth.