The Amber Cat — Hilary McKay

It's December, and Robin, his friend Dan, and Sun Dance are laid low with chickenpox. To keep them amused, Robin's mother entertains them with memories of the summer when she was eleven and Nick and Charley stayed at her house. When they met Harriet, a strange, shabbily dressed girl who seemed full of surprises and magic, but remained a mystery, until Sun Dance solves the riddle with a small amber cat . . .