A good writing friend once shared this piece of wisdom: Sometimes we have to get small to get big, with our books. Book projects span a lot of time and space. It's too easy to get lost in such an expanse, overwhelmed with all the details.
In my writing classes, I used two fun exercises to help writers manage the immensity. One exercise is a poem, the other is an exploration of one of your main characters, your narrator, or your potential reader, by putting them in a five-page short story.
These two exercises are such fun, they can feel like a sidetrack away from the "real" writing. But they give a serious boost to creativity.