It is an adaptation of the "East of the Sun and West of the Moon" fairy tale, following it rather closely, but novelized (lengthened) and with some added flavor.
It follows the tale of a young girl named Camille who agrees to marry a prince she's never met in exchange for him providing for her (very large and poor) family. The prince is suffering from an unknown curse, but even so he is chivalric and Camille falls in love with him. But Camille inadvertently triggers the curse, whisking the prince and his entire estate away on a dark wind. With the help of a mysterious oracle, and other friends along the way, Camille seeks to rescue her prince who is somewhere "east of the sun and west of the moon".
Naturally, this book shares many tropes with East of the Sun and West of the Moon.
Tropes in Once Upon a Winter's Night:
- All Trolls Are Different: They wear animal skins and have tusks and claws, and seem to have some amount of control over goblins.
- Animorphism / Involuntary Shapeshifting The result of a Curse placed upon the prince by trolls
- Bears Are Bad News: inverted. The bear that comes to get her is very nice. Turns out it's the prince himself, as he's cursed to be a bear by day
- Bestiality Is Depraved: The troll princess is actually looking forward to marrying and mating with a bear. It's also implied in the "Where Are They Now?" Epilogue that one of Camille's sisters married a man who forced his dogs on her in such a manner.
- Cassandra Truth: Camille doesn't believe that the tiny Jotun can actually be giant and intimidating. So they trak across the mountains with him in tiny mode. But after he leaves her at the town he grows to his normal height. If she had listened, they could have crossed the mountains much faster, as he said.
- Disproportionate Retribution: Camille makes the single mistake of trying to see her lover's face, and has to spend the next year and a day searching for him.
- Hedge Maze: Alain has one in his garden. Two members the staff are deathly afraid of it. turns out this is part of the curse, keeping them from reaching the center and freeing Alain's trapped parents
- Impossible Task: finding the one place east of the sun and west of the moon in a year and a day, seeing as both heavenly bodies move about constantly...
- Parental Abandonment: Alain's parents have been missing for some time
- Riddle Me This: Camille must answer riddles to receive help from the Fates.
- Taken for Granite: Alain's parents had been turned into the stone statues in the middle of the hedge maze
- "Where Are They Now?" Epilogue: Camille gets one when she is talking to her brother who she hasn't seen since before the ordeal. He tells her (and us) what has become of Camille's family.