I've just finished reading The Cloud Atlas by David Mitchell. It's definitely been added to my top 5 favorite books. (I don't have a favorite book, the top 5 cannot really be ranked.)
It's incredibly imaginative and very well written. It takes place in six different time periods, three of which are in the past and two in the future, while one is roughly the present. Mitchell creates a different voice and medium for each time period's narrative, using historic and envisioned vernacular.
I of course, loved the futuristic worlds he created best of all, including the language he embellished. However, the book is not complete without the tales that take place earlier.
The book is set up so that you only get half of each time segment at a time. We read The Pacific Journal of Adam Ewing first, but it's cut off by Letters From Zedelghem where Robert Forbisher writes about reading The Pacific Journal. But his letters are cut off by Half-Lives: The First Luisa Rey Mystery, in which Luisa Rey meets the man to whom The Letters From Zedelghem were addressed. Her tale is abruptly cut off to find us with Timothy Cavendish who is a Publishing agent reading the Luisa Rey Mystery. The Ghastly Ordeal of Timothy Cavendish becomes a film sneakily viewed by Sonmi-451 in a (not so?) distant future. Sonmi becomes a deity for Zachry in Sloosha's Crossin' an' Everythin' After in a post-apocalyptic world. He finds out about her and then we start going backward in time. The second part of An Orison of Sonmi-451 reveals her truth leading to the second part of The Ghastly Ordeal and so on back to The Pacific Journal.
I think the main theme of the book is not reincarnation as it may seem at first glance. Rather, it's freedom. Simple, yet in each sub-story, it is clear that it is the goal for which everyone strives.
The film did an extraordinary job of bringing the various stories to life and weaving them together. The book seemed to only follow one person's reincarnation, but the movie made it so that the actors were all the same in each segment. They changed race and age and even gender throughout. Brilliant in my opinion.
One essential thing the film did was create Robert Forbisher's Cloud Atlas Sextet for Orchestra. This piece was central to The Letters From Zedelghem storyline and Luisa Rey found a copy of the music. In the film, it is perfectly beautiful and haunting.
As far as how the book compared to the movie, both are excellent and well imagined. Both are favorites of mine. Go read. Go watch.