Many years ago, I read a fantasyish book most probably aimed at children. I remember nothing about this book -book series?- except a character having to eat a green sea apple for some reason, and some other, more vague details. I know this is really not much to go on, but I cannot imagine many books featuring something as original as green sea apples (which I believe were supposed to be small, hard, and sour, though I'm not quite certain). So there it is: Does anyone know what book(s) I'm talking about?