A children's author, novelist and short story writer, she is best known for such works of children's literature as The Silent One (1981) and Bow Down Shadrach (1991). Her novels for adults, including Man of Straw (1972) and The Growing Season (1979), typically focus on family struggles.
Her birth name was Cassia Joy Summers, and she was born in Levin, New Zealand. She received an honorary doctorate from Massey University.
She published five hundred reading instruction books and was actively involved in teaching reading skills to children and the learning disabled.
She was married three times-- to Ted Cowley, Malcolm Mason, and Terry Coles-- and had two sons and two daughters.
Her first novel, Nest in a Fallen Tree, was made into a 1971 film with a screenplay by the renowned children's author, Roald Dahl.