From the official Agatha Christie website:
As early as 1930, my mother wrote her first novel using the name “Mary Westmacott”. These novels, six in all, were a complete departure from the usual sphere of Agatha Christie “Queen of Crime”. Giant’s Bread was first published in 1930 and was to be the first of six books under this nom de plume. It is a novel about Vernon Deyre, his childhood, his family, the two women he loved and his obsession with music. My mother had some experience of the musical world having been trained as a singer and a concert pianist in Paris when she was young.
Her second Mary Westmacott book Unfinished Portrait was published in 1934. It also relied a lot on her own experiences and early life. In 1944 she published Absent in the Spring. She wrote in her autobiography: “Shortly after that, I wrote the one book that has satisfied me completely. I didn’t want to change a word and although I don’t know myself what it is really like, it was written with integrity, with sincerity, it was written as I meant to write it, and that is the proudest joy an author can have. ”I think Absent in the Spring combines many talents from Agatha Christie, the detective story writer. It is very well constructed, compulsive reading. You get a wonderfully clear picture of all the family from the thoughts of one woman alone in the desert – really quite a triumph.
In 1947 she wrote The Rose and the Yew Tree. This was a great favourite of hers and mine too. It is a haunting and beautiful story, I used to write my paper on. The Mary Westmacott novels have been described as romantic novels but I don’t think that is really a fair assessment. They are not ‘love stories’ in the general sense of the term, and they certainly have no happy endings. They are, I believe, about love in some of its most powerful and destructive forms. The possessive love of a mother for her child, or a child for its mother in both Giant’s Bread and Unfinished Portrait. The battle between the widowed mother and her grown-up daughter in A Daughter’s a Daughter. A girl’s obsession with her younger sister in The Burden and the closeness of hate to love – The Burden in this story being the weight of one person’s love on someone else. Mary Westmacott never enjoyed the same critical acclaim as Agatha Christie, but the books achieved some recognition in a minor way and she was pleased when people enjoyed them – she was able to fulfil her wish to write something different."