I am in that lovely stage of writing a book when everything is a piece that might fit. It’s the stage where there is still room for some large elements (like the infestation of naiads and dryads that occurred a few weeks ago; it remains) and a ton of small elements, and so everything is examined for whether it’s the right element.
So when I read a book as background for a future project, I found stuff in that book for three other projects, including the one I’m working on. The one I’m working on is not really a surprise: it’s what my brain is obsessively turning over, as above. But I think I’ve figured out why I keep getting so many ideas for other projects in this stage. Usually I try to approach everything with the attitude of, “How can this be awesome?” And in this stage, it’s a very specific and heightened kind of awesome. It’s not just “that’s a cool thing to know,” but “that fits into narrative in the following known way.”
It’s not that books I read out of this stage won’t contribute to other projects in similar unpredictable ways. It’s that the penny can take much longer to drop. When a book is accreting material like a literary gas giant, it all happens so fast, so I notice it more. That’s all.