Mr SubText him self is writing a ASP.NET book along with Jon Galloway and Jeff Atwood, not saying much on details yet but I am hoping they write a cool SubText book.
We're writing an ASP.NET Cookbook type of thing. I think I'll see if there's interest in a Subtext book after Subtext 2.0 when we have a real plugin API to program against.
When are you going to write a book on MbUnit?
When I get 3.0 out, I might think about it Phil. Expect to see that SubText book next year ;-)