What novels have had such a powerful impact that they triggered an actual change in socio-political policies or overall public opinion on an important subject?
I can only think of two, offhand:
Uncle Tom’s Cabin, by Harriet Beecher Stowe
The Jungle, by Upton Sinclair Any others?
As far as I know, books like Grapes of Wrath may have had enthusiasts, but by the time it came out, it was too late for any relevant policy. And I know that Dickens may have influenced public policy, but I don’t think it was due to a particular novel.