Which 1930 Agatha Christie novel was the first Miss Marple novel?

The 1930 Agatha Christie novel that introduced Miss Marple in a full-length mystery was The Murder at the Vicarage.

Published in the United Kingdom on 20 October 1930, the novel is set in the fictional village of St Mary Mead. The narrator is Leonard Clement, the local vicar, whose household becomes entangled in the murder of Colonel Lucius Protheroe. Miss Marple, an elderly resident with an unusually sharp understanding of village behaviour, helps uncover the truth.

Christie had already used Miss Marple in short stories published in magazines from 1927. However, The Murder at the Vicarage was her first novel-length appearance, which is why it is conventionally identified as the first Miss Marple novel.

Readers sometimes confuse it with The Body in the Library, published in 1942 and often remembered because of its famous opening. That book was the second Marple novel, not the first. The Moving Finger and A Murder Is Announced also came later.