Well, Mark would probably want to write the book himself; he could do it, he can write (not just songs), he was a journalist, he loves literature. But I think it would be very difficult for him to write something like that – precisely because his standards are so high and because he would constantly doubt himself (what to reveal, what people are really interested in).
Writing songs is certainly easier for him, and performing them with such a wonderful band is comparatively child's play; almost nothing can go wrong.
Maybe MK will do something completely surprising one day: why not an absolute solo album, for example — just him, the (acoustic) guitar, and the story, like in the old folk or blues tradition. There's no question that he could do it. I would love it (as much as I love his band).