Well, sort of.
The book is written in epistolary form, from the point of view of four main characters around 5 generations apart. Two characters write in diaries - these are the main characters, and the storyline that the film is based on. To be honest, I saw very little point in the inclusion of the younger two characters, set in our time and seeking to unravel the mystery of their ancestors. I think their parts added a little suspense as more and more secrets were revealed, slowly.
The writing style was very different from what I had expected, but I found it very successful. It made the characters unlikeable and yet also somehow relatable, and I ended up genuinely caring for them, to my surprise.
There are a huge number of plot twists, though some are so foreshadowed that it is hardly a revelation when they are revealed at last - more of a "I knew I was right!" moment.
I wish I had read the book before seeing the film - the film would have made more sense and, in turn, the book would have retained its secrets for longer, I think. However I can definitely recommend discovering both forms of this story - it's a good one.