[Combwork]

Originally Posted by finnbow

From Wiki:

By the 1900s, nigger had become a pejorative word. In its stead, the term colored became the mainstream alternative to negro and its derived terms. Abolitionists in Boston, Massachusetts, posted warnings to the Colored People of Boston and vicinity. Writing in 1904, journalist Clifton Johnson documented the "opprobrious" character of the word, emphasizing that it was chosen in the South precisely because it was more offensive than "colored."[9] Established as mainstream American English usage, the word colored features in the organizational title of the National Association for the Advancement of Colored People, reflecting the members’ racial identity preference at the 1909 foundation. In the Southern United States, the local American English dialect changes the pronunciation of negro to nigra. Linguistically, in developing American English, in the early editions of A Compendious Dictionary of the English Language (1806), lexicographer Noah Webster suggested the neger new spelling in place of negro.[10]

By the late 1960s, the social progress achieved in US society, by such as the Black Civil Rights Movement (1955–68), had legitimized the racial identity word black as mainstream American English usage to denote black-skinned Americans of African ancestry. In the event, the “political militant” connotations of black displaced it in favor of the compound blanket term African American. Moreover, as a compound word, African American resembles the vogue word Afro-American, an early-1970s popular usage. Contemporaneously black Americans use the word nigger, often spelled in eye dialect as nigga and niggah, without irony, to either neutral effect or as a sign of solidarity.

Sounds about right to me.

Me too. As Mark Twain published his book in the late 19th century, I guess he just used a word that was common parlance and not seen by everyone as insulting. As for Agatha Christie, in the 1930's there were few black people living in England, so I guess she just picked a word out of a nursery rhyme.