While we're on the topic of music, I recall Josh or Sophie having an iPod in the Yggdrasil that was drained of power and later destroyed along with the tree (or perhaps it was a laptop, I'm not so sure). What sort of music do the twins listen to?
Unrelated to music now, I was wondering when you write, how much do you usually do at a time or per day? Because you see, I know of some authors that write a lot and as such get books done fairly quickly (Matthew Reilly), and some others who take their time to finish a book (Chris Paolini). Although I think it goes without saying that an 80,000 odd word plot summary helps to keep you going.