The Lord of the Sands of Time

The aliens have invaded in ancient Japan - but help is at hand from a time travelling AI from a future (or possibly parallel) Earth.

This book jumps between the story set in Japan of the past, and the alien invasion in the future. The only hope for the future is to fight the aliens in the past, and so O, aka The Messenger, comes to Japan as the voice of the gods, and sets out to save the future.

Fun story, though quite short. I particularly loved the parts set in old Japan.