by Joan Aiken is a good book.
The main characters are:
Sylvia Green, Bonnie's cousin
Simon, a friend of Bonnie's who lives in the woods
Miss Slighcarp, Bonnie's fourth cousin
Mr. Grimshaw, Miss Slighcarp's friend
and Mrs. Brisket, another of Miss Slighcarp's friends.
P.S. There are lots of sequels.