"Return to Gone-Away" by Elizabeth Enright (author), Beth and Joe Krush (illustrators) (Gone-Away Mysteries)

Portia Blake's wildest dreams have come true -- her family is moving to the Villa Caprice, the old mansion once owned by Mrs. Brace-Gideon, an eccentric millionairess! There she'll be neighbors with her cousin Julia, and also Uncle Pindar and Aunt Minnehaha, the last residents of Gone-Away, the ghost town on the lake that disappeared.

When the Blakes actually go in the Villa Caprice, it's more than they imagined. Mrs. Brace-Gideon left behind a fortune in antiques and curiosities, some very strange and spooky things. On top of it all, the Villa has a big attic; there's a lot to explore there, if they dare!