premise: The Lonely Londoners is a 1956 novel by Tamil Trinidadian author Samuel Selvon. Its publication marked the first literary work focusing on poor, working-class blacks in the beat writer tradition following the enactment of the British Nationality Act 1948.
hypothesis: The Lonely Londoners was motivated by the British Nationality Act.
reason: It says in the text that the book was spurred on by the enactment. I simply rephrased this in different language.