Eon

Above our planet hangs a hollow stone, vast as the imagination of Man. The inner dimensions are at odds with the outer: there are different chambers to be breached, some containing deserted cities; the furthest chamber contains the greatest mystery ever to confront the Stones scientists...

But the Stone is not an alien structure. It comes from the future of our humanity. Tombstone, or milestone, the war that breaks out on earth beneath its presence seems to bear witness to its prowess as oracle...

In the near future when human kind is, once again, on the brink of total self-destruction, when The Stone arrives. The Stone is a hollowed out asteroid; containing seven chambers each with their own landscape and cities. The clincher is that it seems to be constructed by humans! It could be from our future or it could be from some other future, the only thing certain is that there's a library that describes the near total destruction the coming war will bring, and the long nuclear winter that will follow it. The race is on – is the asteroid from our future or from the future of some less lucky parallel universe?

Bear has taken one really good idea and spun a very interesting story around it. Most of it is very well written and most of the characters are if not likeable then interesting and real enough to work.

I did find the parts about the ...hm... future inhabitants of the Stone rather uninteresting – I didn't really care about their internal politics. But I'll still say that this is probably one of those books whose basic idea is so good that, nearly no matter how boring you will find one of the subplots, you just have to love it.