A few years ago we found some interesting books at Barnes & Nobel. The series, written by Adam-Troy Castro, is named after Gustav Gloom, a boy who lives in a strange mansion.
Fernie What moves in next door and watches as her cat is chased into the mansion by its own shadow. Fernie meets Gustav, who cannot leave the mansion, and they become friends and must defeat the People Taker before he takes Fernie's family. The story is full of spooky illustrations by Kristen Marigotta.
The series has grown, I've read the next two but there are five total so far:
It's been a couple of years since I read the books, but I remember that there is something about Gustav that ties him to his home. I think he is 1/2 spirit, 1/2 living. The spooky stuff is mild, so these books would be fine for most third and fourth graders.
I'd like to get my hands on the last two books, I really enjoyed them. Often children's books seem to follow cliched patterns, but the Gustav books have a lot of new and interesting ideas in them. They are spooky and refreshing at the same time.