Why not simply name the book after her, if she is such an important character, or maybe name it ‘Elfland’s Princess’ or something similar? After reading it, I think you’ll understand why a more direct title is unsuitable for this fairy tale: the titular king of Elfland’s daughter is not so much the protagonist of the story as the catalyst.
Phantastes Review
I can see where there would be a certain resonance perceived with Lewis's The Lion, the Witch, and the Wardrobe, since the road to the land of the fairies in Phantastes also passes through a piece of old furniture, in this case a desk.
IGC Publishing 