I Am Mary Dunne is a novel

Novel

A novel is a book of long narrative in literary prose. The genre has historical roots both in the fields of the medieval and early modern romance and in the tradition of the novella. The latter supplied the present generic term in the late 18th century....

 by Northern Irish-Canadian writer Brian Moore

Brian Moore (novelist)

Brian Moore was a Northern Irish novelist and screenwriter who emigrated to Canada and later lived in the United States. He was acclaimed for the descriptions in his novels of life in Northern Ireland after the Second World War, in particular his explorations of the inter-communal divisions of The...

 about one day in the life of a beautiful and well-to-do 31-year-old Canadian woman living in New York City with her third husband, a successful playwright. Triggered by seemingly unimportant occurrences, the protagonist

Protagonist

A protagonist is the main character of a literary, theatrical, cinematic, or musical narrative, around whom the events of the narrative's plot revolve and with whom the audience is intended to most identify...

 / first person narrator

Narrator

A narrator is, within any story , the fictional or non-fictional, personal or impersonal entity who tells the story to the audience. When the narrator is also a character within the story, he or she is sometimes known as the viewpoint character. The narrator is one of three entities responsible for...

 remembers her past in a series of flashbacks

Flashback (psychological phenomenon)

A flashback, or involuntary recurrent memory, is a psychological phenomenon in which an individual has a sudden, usually powerful, re-experiencing of a past experience or elements of a past experience. These experiences can be happy, sad, exciting, or any other emotion one can consider...

, which reveal her insecurities, her bad conscience

Conscience

Conscience is an aptitude, faculty, intuition or judgment of the intellect that distinguishes right from wrong. Moral judgement may derive from values or norms...

 concerning her first two husbands, and her fear that she is on the brink of insanity

Insanity

Insanity, craziness or madness is a spectrum of behaviors characterized by certain abnormal mental or behavioral patterns. Insanity may manifest as violations of societal norms, including becoming a danger to themselves and others, though not all such acts are considered insanity...

.

Of all of Moore's books, I Am Mary Dunne has been described as "perhaps his best novel" in The Cambridge Guide to Literature in English (ed. Ian Ousby, 1988).