The term "world-building" tends to conjure images of maps and manuals and sprawling, fantastical lands. But the worlds that interest me most are usually enclosed by four walls.

I've always been fascinated by houses, both in reality (I'm shamefully prone to peering through neighbors' lit windows after dark) and in fiction, from Green Gables to Hill House. My own first story for young readers -- the story that eventually turned into The Books of Elsewhere -- was inspired by a house in my hometown: a once-grand Victorian that now sagged over a lawn full of fantastical wind-powered machines.

Why do I find houses so endlessly fascinating? I think it's because a house is a world.

A house has its own climate and atmosphere. It has its own inhabitants and culture, its particular food and art and rituals. A house has its own history, its own rules or ethics or powers-that-be. And (like another beloved fictional structure), houses are bigger on the inside -- big enough to contain most or all of the action of an entire novel.