When I was...a junior, maybe a sophomore...in high school, I hung out with this particular senior. I don't think we had anything in common besides a love of Julian May books, but that isn't the point. He lived on Beacon Hill, a place that circumstances had me near all the time, and he dragged me around one night. He showed me his job--some shop that I no longer remember--and took me into the cellar to show me the tunnel that had been used first for the Underground Railroad. It was, for a few feet in, anyway, just extra storage by then, but he said that there were lots of secret holes and tunnels in odd places about the Hill. He had me at "secret."
My mind would wander to those tunnels over the years, wondering if they connected to the subway at any point, if Fairies had moved within like sidhe mounds now that they've been abandoned. I've searched the net--you know, now that it's decades later and ordinary people can search the net, for info that's never there.
Beauty and the Beast - The Complete Series introduced me to ordinary (and in Vincent, not so ordinary) tunnel dwellers back in 1990, while Gaiman's Neverwhere: A Novel made the idea more magical, and took me to the rooftops and over suspension bridges in 1998.
This morning in the shower, for no reason in particular, I imagined a Boston where the fay rule the underground, urban shape shifters mingle with humanity in the streets, and vampires and angels fight for supremacy in the skyscrapers.
That's usually how it works for me. Rather than a lightening bolt of inspiration that develops into a book, I have a spark, that might sit in my mind or in a notebook for years, meeting other sparks along the way until I have something to work with. I have built stories from flashes of inspiration, but those are all waiting for pieces to make them whole, while the book that I'm revising to eventually send out came from bits and pieces collected over a good handful of years.