Young Adult Fantasy Novel?

I can't remember title or author. It was about a princess whose brother is away at war against the gnomes when the castle is attacked so she runs away to warn him. Along the way she meets a gnome who she saves the life of so he is in her debt and agrees to guide her to her brother. She also meets an Ent and eventually ends up begging for the gnomes life in front of his king because he helped her, which fulfills a prophecy about a sun weeping in the dark caverns. Anyway I really enjoyed and can not find reference to it anywhere any help would be lovely.