There is a corollary for "How's the book coming?" The polite response is "Fine, thank you." The one I usually give is, "Uh....it's coming along slowly." But even that doesn't begin to describe that mess that is my office, my head and my manuscript.
The most daunting thing about writing a book - at least to me - is not the writing, or the research or the self-discipline involved. It is the complete lack of feedback. I am used to writing for a newspaper, where every paragraph you write is immediately read by an editor, who sends it back with questions and comments and generally guides you to where you need to go next with your work.
In this world, I write for months on end, without having the slightest idea if what I'm doing is any good or not. OK - that's a slight exaggeration - my agent is reading the first few chapters and says that when they are in decent shape (which evidently they are not yet), she will send them to my editor at the publishing house. And then if I'm really lucky, she can secure me an extension, which I will no doubt need.
Oh, sorry. Did you ask how the book was going? "Fine, thanks."