They're simple little books, but they're such a great adventure. I read these when I was growing up and even now, reading it as someone who's almost 25, the world and the ideas floating around in the background are really fantastic. Like seriously, what other book can you name that puts a mermaid in a lake in the tunnels of a volcanic mountain? Or a gate to a hidden place in the desert that can only be seen when viewed using water, either covered in it or in the reflection of it? They're just such great kids books.