The story built up quite quickly to a point where I really just had to know what was going to happen, especially with the famous letter that was the center of most of the main characters' stories, with the 'past' storyline about delivering the letter and the 'future' storyline about finding it. I also really liked the setting of both the parts of the book in the 'frontier' era of a slightly different past than in our own universe and the post-apocalyptic future, with Big Brother-like ruling class.
The build-up of the story was very tense and really made you feel the stress the characters were going through. Unfortunately the ending had very little pay-off for all this build up. It resolved almost no part of the story, and never actually clarified this mysterious letter that was such a big part of the story. I was glad to have read it, but the ending really disappointed me. However, if you're not a person who prefers (needs) a very concrete, conclusive ending to your stories, then I think this wouldn't be as big of a problem. Overall, I do think this is a book worth reading as the story and format are so unusual and interesting.
Keep reading! Beth