Librarian to the Rescue. Tilly, What Have You Done Now?
by David and Linda Broughton
Two sisters, fraternal twins but nothing at all alike, accidentally walk into a burglary in progress, then things snowball from there. Tilly is three times normal size, in every way, while Martha, the librarian, is small and mousy. The tide turns, out of necessity... then there's more trouble to deal with, as Mousy Martha becomes Mean Martha, the Godmother. Humor mixed with mystery and action.