I read this book some years ago, probably around 2010, though it was definitely older than that. It had a fairly short and simple title, of the kind one might expect to find on a fairy tale: "The Lost World" or "The Princess's Journey" or something in that vein. My copy was hardback, the cover mainly white with a stylised drawing running across both back and front, featuring a few people, one rather prominently on the front, and a landscape with mountains.
At first I thought it might have been written by Philippa Gregory, but I checked through her bibliography and couldn't see it there. The author's surname definitely began with an early letter in the alphabet (A to H) - I remember that from where the book was in my bookcase - and it was probably written by a woman. Edit: on further reflection, I think the author may have been called Maria, with a surname starting with G and possibly Gl.
I don't actually remember much of the plot. It was a typical fairy-tale-ish type story: a princess going on a quest? meeting a poor boy in a forest? that kind of thing. The really memorable and distinctive part was that the story was told twice in the same book: once in a relatively short and simple way, prefaced by "this is the story of [title of book] as I first heard it", and then again in a longer way with more details included, prefaced by "this is the story of [title of book] as it was told to me when I wanted to know more". In their essential details, both stories were the same, but there may have been some actual differences between them as well as just extra material in the second retelling.
What was the name of this story? I'm sure I'll know it when I see it, but it's still eluding me.