When I was a teenager, I read a really good medievalesque fantasy trilogy just before we moved into a new house: Cup of the World, by John G. H. Dickinson. In my new bedroom, the old owners had left a paper-mache moon globe around the ceiling light. So, I named it after the coat-of-arms of one of the main characters. Except, his was a moon with a bite out of it, the "Doubting Moon," which brings up rich symbolism about what's shadowing the moon. My own moon, I accordingly named the "Doubtless Moon."

Last month, I reread the trilogy, and it's even better than I remembered it. It's well-told, it gives an interesting interpretation of medieval Europe through a fantasy counterpart world, and it keeps exploring the rich thematic questions it raises without ever ducking or (to use the term I talked about earlier) rigging them.
The series starts with a decoy genre: it appears to be a teen romance with our young protagonist Phaedra in love with a mysterious man appearing in her dreams, who finally appears in real life to rescue her from an impending arranged marriage. The kingdom she lives in is portrayed unsavourably; we open with her watching a witchcraft trial which appears to be a mere exercise of power. We can easily think that the witchcraft is made up, or unfairly slandered, as it would be in many other fantasy books.
Yet afterwards... she's very much in love with her new husband (the man from her dreams, who turns out to be the lord of Tarceny), but her rescue spawned a war. He wins surprising victories in the war, but he's hurt by a reputation for witchcraft, and rumors that his triumphs were through witchcraft...
... And then it turns out, the rumored witchcraft is real. And, it really is about as unsavory as people in the book claim. The power that he's dealing with demands harsher and harsher deals, taking away more and more before betraying him. One old review (by a now-defunct blog) said the book is "upholding such good old-fashioned principles as Thou Shalt Not Make A Bargain With The Devil Or Any Reasonable Fascimile Thereof" - and I do love the excellent picture of temptation it gives.
There's a temporary triumph at the end of Book One; the power behind the witchcraft comes to collect on the penalty clause in his bargain, and things seem to be stable for the moment. But, it's an uneasy stability: the power of witchcraft is tempting. At the start of Book Two, someone else makes a bargain for power, and wars start up again.
The scene expands in Book Two to show us more of the kingdom, and the one university that's survived the constant wars. There, we see the same facsimile of the devil tempting not to power but to despair. On the one hand, Dickinson shows us a pastiche of medieval philosophy which can sound hopelessly abstruse. (And if it sounds hopelessly abstruse to me, who's read a bit of actual medieval philosophy, what must it sound like to the average reader?) On the other hand, from how Dickinson frames it, he obviously values the hope of a better world it gives (similar to that of real medieval philosophy). Here, his vision for the story isn't quite born out on the page.
And then in Book Three, the scene expands again, and the thematic questions expand again. The devil-faximile has been done away with at the end of Book Two, which would seem to end the thematic exploration. Except, it doesn't. Instead, it becomes clear the central temptation wasn't just witchcraft after all, but the yearning for power itself. We see it corrupt our protagonist, Amba, son of the Phaedra from the first book. Even after the throne has been offered to him in peace, he feels he needs to keep it by war. Instead of ducking the thematic question, Dickinson answers it in an even richer way than I'd expected.
Another review criticized the book for its religion being an uncreative expy, or barely-altered copy, of medieval Catholicism. They're not inaccurate, in that it is a clear expy - but I think that plays into the setting's interpretation of medieval Europe. Dickinson's "Kingdom" (unnamed) has reduced medieval Europe by taking out many elements I'd say were important, such as the free cities, the monastic orders, the technological developments, trade routes... But those are peripheral to the point he's making. We get hints that some things like that are happening, but that's not the point.
Dickinson is taking medieval warfare and drawing it to an extreme. His kingdom's constant wars have reduced the kingdom's great houses, laid land to waste, and (we see in Book Two) ruined every university but one. Yes, they have been explicitly under attack by a hostile metaphysical power. Yes, you could explain how they've done worse than actual medieval Europe by pointing to that. Some hints are given that's not all, but it's part of it. Still - drawing this extreme lets Dickinson hint about actual historical wars.
And, in Book Three, we see the wars come back even after our protagonists have somehow managed to make peace. Even though the power behind the witchcraft has been defeated, the yearning for power is inherently corrupting. We see it in Amba our protagonist, which makes us remember how we've seen it in so many other people throughout the books.
Dickinson's central question - thought experiment, as it were - evolves through the trilogy. We start out with how Phaedra should respond to her husband's rumored witchcraft; then it becomes real witchcraft. Then (in Book Two) we turn to how to respond to witchcraft-fueled wars and despair in general. And in Book Three, the question expands again to how to deal with the hunger for power?
But we end with a broader version of the same question we started with, which - we realize - was the same question we should've been asking at the start. Dickinson hasn't rigged his thought experiment <>. He's answered the questions, and then turned to the broader question we should've been asking at the beginning.
Tolkien did something of the same thing in The Hobbit. We start out The Hobbit with a light-hearted kids' adventure story; we end with an exploration of greed and a bittersweet victory. Bilbo has returned to the Shire changed; the story has reached its ending changed. Bilbo isn't the same hobbit as at the start of the story, nor is the story the same story. Someone who liked the old story more could certainly fault this, but it's organic growth rather than changing the original "thought experiment".
Here with Dickinson, I'm one of those people. I do like the first two books better than the third. In part, this's because it's painful seeing Amba brought down by his own choices as king: his own misguided trust, his feeling he needs to keep the power he's gained, and the folly of the people he's trusted. And then, after that, there's no clear victory or even clear defeat. The ending is uncertain.
I'm sure Dickinson would say this's only what's realistic. I have to agree. In the real world - both the Middle Ages he's mirroring and most other times through history - most endings are unclear and people are often brought down by merely their own choices. What's more, when writing about power corrupting a good person, that's a very realistic way to tell the story.
I've seen this story before. I've seen it recently, in Gillian Bradshaw's retelling of the King Arthur mythos. Just like here, I didn't like it, and the trilogy that had been excellent for the first two books dropped to merely good (and painful) in the third book. Just like here, it was perfectly on theme, and even more warranted since Bradshaw was weaving a tale through the existing Arthurian mythos rather than making her own story like Dickinson is. The tale has turned into a tragedy, having subverted its hints by not subverting them after all, and a tragedy with characters I've already grown to know and love.
But despite the painful ending, the tale is told well. Dickinson echoes earlier points of the series beautifully, and we see just enough of what could have been and might still be if the renewed wars do not continue.
In the end, The Cup of the World trilogy is a rare work of fantasy that dares to let its themes evolve to a genuine end, even when it turns the initial story inside-out into a tragedy. But it does offer clarity, in the end. Rereading it now, I found it held up even stronger than when I'd first read it. It'd grown with me, still asking real questions and finding deeper shadows and, if not complete answers, deeper answers than the initial story seemed to herald.