The novel is a concatenation of six stories which span a huge period of time from the 19th century to way in the future. However, each story is a cliff-hanger, ending abruptly before passing on to the next. Initially, I found this disconcerting and slightly annoying, as you're plunged into another time period with an entirely new set of characters and you have to find your feet again. But as the book progresses, you start to recognise the subtle resonances and reverberations that thread the stories together, and the jump to a new time becomes an adventure. About three-quarters of the way through the book, it reaches a climax, and you cascade back through the lives of the six characters again, finding out how they ended (or how it all began, depending on your viewpoint).
Mitchell has an amazing facility for writing in a very natural way in a huge diversity of styles. The first story — the sea journal of Adam Ewing — is written in a fastidious and slightly prissy voice, which sounds (to me at least) totally authentic, then there's the dry wit of a composer in the 1930s and so on. At one point (and I really don't want to give anything away here), he invents an entirely new dialect, which seems to be have a consistent grammar and vocabulary. Despite these slightly virtuoso touches, he never gets too clever. The links between the stories could be too complete and perfect to be believable, but they're not.
It's one of those books that you desperately want to describe and explain in detail to others, but at the same time, you don't want to spoil their excitement when they read it for the first time. All I'll say is that it's broadly about the dangers of greed and the lust for power, and the havoc they can wreak. One phrase — spoken by one of the characters — has stuck with me: "Before The Fall was tripped". Falls (with a capital 'f') are rarely entirely accidental, and usually someone has stuck a metaphorical foot out to precipitate it. This book can be seen as a parable about how we humans can avoid tripping.
P.S. I think that this is one of those books that polarises opinion: you'll love it or hate it passionately. Obviously, I'm in the former camp.