Arthur C. Clarke is one of the greats of science fiction. He is a master of that genre of sci-fi in which one takes A Big Concept and works through the consequences. This is such a novel, but with an odd twist - you get a historical drama as well. In this story, an alien race, whom we never actually meet, messes around with earth such that bits of the planet from different time periods suddenly find themselves coexisting - a prehistoric humanoid finds herself abruptly in the same world as a British fort on the Northwest Frontier in the times of the Raj and they are startled by the arrival of a military helicopter from the early 21st century. Simultaneously, some astronauts due to re-enter the atmosphere find that the world they knew has ceased to exist.
What Happens Next makes for fascinating reading. Attached to that British garrison is Rudyard Kipling. And at the climax, the greatest captain the world has ever seen, Alexander the Great, faces off against the most ruthless conqueror the world has ever seen, Genghis Khan. All a bit contrived, to be sure (the manner in which Genghis gets his comeuppance is especially hard to believe, but then I am a devout coward by religion, so what would I know?). However, it's all good fun. And all of this is observed by aliens in the form of shiny spheres, which hover around, observing what happens.
In a way, this book is a rerun of themes explored in the Clarke classic "2001; a space odyssey", and those who have read "2001" will enjoy the quotes and references to it in this book.