We are always working on the book. Not to be at work on a book, at a Research 1 University, means not showing up for work, since 40% of our effort is supposed to be research. Yet the book is elusive. There is the first book, that is either the dissertation or a second project written very quickly before tenure. Then there is the post-tenure book, but that might be the last one. People retire with two books, and no more, and three is considered great. What this means is that, really, many people do very little scholarship after the dissertation, and there are often long fallow periods.
So the book is in many cases a fictional one, not a work of fiction one that will never exist at all. I probably wouldn't have the courage to say that I'm not working on a book. I couldn't say: Oh, I've published 5 and will spend nine years waiting for retirement; I've already done enough.