Geoff Ryman wrote 253!

I almost forgot about this interactive novel set on a London tube train.
He's the co-Guest of Honor with Ellen K for WisCon 2009.

Note in 2009 - I can't believe I forgot about this. Again, in the WisCon 2009 posts, not sure when would have been the best place to talk about it - probably in the panel discussion about his work but I didn't make it to that either.

I really liked this novel.