What's your metaphor for your book? Is it a scale, where two things have to be balanced on either sides? Do you think in terms of surface and depth or in terms of "coverage" of a certain amount of territory? Is it a miscellany or a coherent whole? An atlas or a triptik? Does it have thread or a trajectory, is it a textile or a rocket? Is it a series of "slots" that can be filled with the appropriate content? A group of concentric or overlapping circies?
Each of these metaphors has its own implications. I am not recommending one over the other, but I (personally) spend a lot of time thinking in and about metaphors for what I am doing. They can be very useful for clarifying what it is you're attempting to accomplish. You'll probably find them shifting as you work. Some metaphors might be making it harder for you to do your work, so shifting to an easier one might be just the ticket.
You could also metaphorize your personal relation to your project. Is it a straitjacket, your best friend, the rock of Sisyphus? Is it a salt mine, a gold mine, or no kind of mine at all? Is it a crop you are cultivating or a game you are playing, a race you are running, a war you are waging? It could be a wild horse you are trying to tame.