I have read and enjoyed many of the classics that others have mentioned. I would give a shout-out to Wasteland of Flint by Thomas Harlan. I enjoyed it for how it incorporated alternate history in our relatively recent past - imagine a world in which the Inca and Imperial Japanese kingdoms became the dominant superpowers and led the industrial revolution, Europeans were conquered and fast forward into far future world with aliens, FTL space travel all mixed with what the Inca and Japanese cultures might have evolved into in that context and how those of European descent were seen as the lower caste. Some interesting cultural reversals and twists mixed in with future technology and ancient alien technology.
