Co-authored and it's got a very wide scope to it. There isn't a single topic covered by science fiction in the last few decades that isn't, at least, hinted at somewhere in the book. At the moment, the working title is 'Torment', but I'm certain we're going to change it. There is, at the moment, at least 20 characters: false protagonists, hidden protagonists etc, but every single one of them is used to death. 73k words atm.
One thing that seems to happen is that my co-author writes, I write, I edit, and he reviews. I don't know who's got the hardest job!