This is the sequel to a book called The Half-Men of O, which I have not read, and there is a bit of a learning curve as the author does not take much time to introduce the setup. That being said, the setup is pretty standard. A boy and girl from Earth were transported to a fantastical world where they saved everyone from an evil tyrant.
But the setup of this book a lot more interesting. When Susan Ferris and Nick Quinn return to the world called "O", they find that it's been a hundred years since their previous visit. In that time, the people of O have built an entire religion based on Susan's heroics, and that religion has become the facade of a new tyranny.
Determined to undo what she now blames herself for, Susan sets out to find a way to confront the priests who oppress the people in her name.
This story was an interesting take on the aftermath of the classic quest-to-overthrow-evil, examining the consequences of how heroics can be manipulated to gain and maintain power.
The characters were interesting, and with the heavy-lifting in terms of worldbuilding done in the first book, the author was able to get deeper into the interactions between characters and societies. The villain was better than expected, even though he doesn't appear until very close to the end. The author made good use of the concept of manipulating religion for political gain, creating an adversary who is more dangerous because he does not believe anything of what he preaches.
I believe there is a further installment in this series, although this book ends at a pretty good stopping point, with some sentimental moments as Susan Ferris says her goodbyes to the world of O