It's very hard to give Cloud Atlas a genre. It's not science fiction - although there are science-fictiony elements. It's not historical fiction - although there are historical elements. It's not a thriller - although there are thriller elements. I suppose it's a mixture of genres, all thrown in together to boil up something new.
There are six narrators in this novel, covering a time from, oh, late Victorian I'd say, to a post-apocalyptic world resembling, in some ways, the one in Stephen King's Dark Tower series. That sounds odd, doesn't it, but all these narrators are linked, cleverly and subtly.
What's it about? Of course, with such a range of narrators, the plot is impossible to describe without major spoilers. But, generally, it's about fighting the darkness, in incarnations great and small, for a better world.
The writing is masterful - Mitchell uses a number of genres and dialects with skill and lyricism, and there are some wonderful quotes in there: "Souls cross ages like clouds cross skies". There is one part near the end, where Ewing works out that he's been poisoned on the basis that nobody's allowed to see him - I'm sorry, that's just stupid for so many reasons that feels massively contrived for the sake of the narrative; but that is only one part in an entire novel that could have gone wrong in so many ways.