Went to a talk given by Darren Shan last week. Felt slightly embarrassed being the only adult there without a kid on tow, but never mind.
The guy's a steam-engine of productivity and said he usually has three books on the go, writing one, editing another, and something inbetween with the third.
Does anyone else work like that? Must admit, I'm not sure I could keep it all in my head. My methodology is finish one, work on another, stop, go back to the first, stop, go to the second. Actually, now I think about it, it's quite a shambles. Basically I'm a fair chunk into Book 2, but will shortly be putting that aside for a few months to rework The Devil's Kiss. What's great with having a sequel is that you don't get confused with you you're writing about since they're the same person, and each book feeds back and forth from the other.