I have been searching for this for years! I got it out of the library when I was 12 or 13 so about 15 years ago now, but I have no clue when it would have been released and my family have no idea what I'm talking about.
All I can remember is that it had quite a plain white/grey cover and the title was something to do with dreams or dreaming?
What I can remember of it was that there were two main characters, I think it was told from two points of view, a boy and a girl and they met in a dream world. There was a journey involved - either a quest type of thing or a race maybe? They were preparing for a battle or fight at the end of the book, so it's possibly a series or duology.