I'm no writer, but it might help to not think about writing a book. I imagine it constrains the work in an unhelpful way. I'd think about describing/communicating something with words. The book is just one potential artifact should the description be a certain size and warrant sharing.

Like, if I were an actor, I wouldn't think of my task as starring in movies. Where would I start in that case? Every small step I made toward the goal would feel worthless, pointless. Instead, I'd think of my task as pretending well. It overwhelms the task to constrain yourself to a particular outcome.