The 'what would you do if you could live your life again?' scenario has been well explored, but this is a good take on it. (The recent film 'butterfly effect' is a recent example of this genre).
The central two characters get the chance to relive their lives not once but many times, but each time their 'life-span', and the point in their lives where they re-emerge after death is shorter. This is an exploration of how things could have been different for them, their friends and family, and the world in general. It is the story of their discovery of what is happening to them, but also what it allows them to discover about themselves. With the history of (mainly) the US since the 1960's to play with there is plenty of material.
One episode, where an increasingly desperate US attempts to intervene in world affairs based on their 'knowledge' of future events, only to see it spiral our of control as events diverge from previous history with terrorism and increasing attacks on the US has some relevance to recent events.
This is a book I have read several times now. Although at times it is in danger of becoming bogged down in the scenario he has created, the next 'replay' is never that far away, so it keeps you hooked to the end.
Some of the questions it poses are almost too big for the book. Can a relationship last more than one lifetime? What are the things that really matter in life? Can we really make a difference to other people's lives?
I heartily recommend this book.