Wishy-Washy.She started out writing novels for adults, and her first book, Nest in a Falling Tree (1967), was adapted for the screen by Roald Dahl.
It became the 1971 film The Night Digger.
Following its success in the United States, Cowley wrote several other novels, including Man of Straw (1972), Of Men and Angels (1972), The Mandrake Root (1975), and The Growing Season (1979).
Typical themes of these works were marital infidelity, mental illness, and death, as experienced within families.
Cowley has also published several collections of short stories, including Two of a Kind (1984) and Heart Attack and Other Stories (1985).
Today she is best known for children's books, such as The Silent One (1981), which was made into a 1985 film.
Others include Bow Down Shadrach (1991) and its sequel, Gladly, Here I Come (1994).She has written forty-one picture books, such as The Duck in the Gun (1969), The Terrible Taniwha of Timberditch (1982), Salmagundi (1985), and The Cheese Trap (1995).
The Duck in the Gun and Salmagundi are explicitly anti-war books.
She has been actively involved in teaching early reading skills and helping those with reading difficulties, in which capacity she has written approximately 500 basal readers (termed reading books in New Zealand).