'How's the book going?' is the question we all ask when someone's reading, or writing a book. Not a book, but 'the book'. How's the book? Funny how the book takes on a status for the time of its being read or written, a bit like 'How's the dog?' or 'How's the baby?' There's only one, and everyone knows what we're talking about.
It's to do with how the book occupies a central space in our consciousness while we're reading or writing it. In my mind I visualise it as centre stage, almost as if it were on altar or pedestal of some kind which the rest of life tiptoes round. Because the book is always with us, if it's powerful enough, just as a child or partner is, even when we're away from it.
How's the book? The book is engrossing or tedious or brilliantly written. The book is a companion and a tormentor. I can't get enough time with it, or I have too much - and the same applies to a book whether I'm reading or writing.