Evolution's Shore by Ian McDonald
[This book is titled Chaga in some countries.]
In this story, Saturn's moon Hyperion has disappeared. Then things fell like meteors to Earth from space. They've landed in areas near the equator. Of these, the book primarily deals with Africa. Starting from its impact point in Kenya, an expanding region of an alien growth is covering everything in its path. Military attacks on the spreading stuff may slow the expansion a little, but nothing more. The Africans have named the alien stuff "chaga".
Chaga does not cover the land in a form like buildings or fields of grain. It is like a tall, lacey sculpture that varies from place to place, or maybe a bit like a jungle if one includes all the different kinds of plants that make up a jungle ecosystem. But it's not entirely clear that this is a plant. In its ability to take over whatever it finds and to produce numerous types of structures possessing particular functions, it is reminiscent of Von Neumann nanomachines programmed to build a city or an equally complex system of diverse parts. In any case, I can't do justice to the book's descriptions of the chaga. It is a strange and fascinating thing.
Mostly, this unfamiliar thing transforming the known into the unknown has caused fear and an urge to keep a distance from the chaga. And if the local people aren't fleeing from the chaga on their own, the UN troops deployed around the edges of the chaga region will make them evacuate.
However, there are rumors of people who live in the chaga. Reporter Gaby McAslin finds a man who has lived in and had his needs taken care of by the chaga. The chaga has acted as if a designed environment for him. What is more, the chaga seems to be slowly transforming him.
This is neither a traditional bad-aliens-invade-Earth nor good-aliens-save-Earth story. I suppose different readers could see it from one perspective or the other. Which is not to say the book has no leaning, just that it gives more food for thought than a book that goes further in directing you what to believe.
It's good reading with excellent descriptions and things to consider.
One can ask interesting questions here. The governments of the world are responding in the panicked way such institutions do when faced with a threat to the status quo. We have no reason to believe they've really thought things through and drawn the wisest conclusions. But let's give the devil his due. The chaga has arrived unannounced. There isn't even an unsupported claim by an unknown stranger from space that it isn't harmful or hostile. So what would be a reasonable reaction by governments?
Whether the chaga is plant, animal, machine or none of the above, its ability to have healthy interaction with Earth life is very improbable unless some intelligent planning for this was involved. However, such questions related to its origins are not really part of the story.