Octagon Magic by Andre Norton
Canadian Lorrie Mallard is sent to live in America with her aunt Margaret, but has a hard time adjusting. She finds her way to Octagon House, home of the elderly Miss Ashmeade and her servant, Hallie. With the help of a rocking horse and doll house she finds in a mysterious room, Lorrie travels back in time - and discovers the Octagon House has always been a refuge for those in need. This volume is one of a unique series of books in which ordinary kids find themselves caught up in extraordinary circumstances, when they are transported to historical, mythical, and legendary settings where they encounter magics of different traditions.