Time's Eye: A Time Odyssey Book One by Sir Arthur C. Clarke
1885, the North West Frontier. Rudyard Kipling is witness to a bizarre encounter between the British army and what appears to be an impossibly advanced piece of Russian technology. And then to a terrifying intervention by a helicopter from 2037. Before the full impact of this extraordinary event has even begun to sink in, Kipling, his friends and the helicopter crew stumble across Alexander the Great's army. Mankind's time odyssey has begun.
It is a journey that will see Alexander avoid his premature death and carve out an Empire that expands from Carthage to China, beating the time-slipped army of Ghenghis Khan in a battle outside the ruins of Babylon in the process. And it will present mankind with two devastating truths. Aliens are amongst us and have been manipulating our past and our future. And that future extends only as far as 2037, for that is the date Earth will be destroyed.
This is SF that spans countless centuries and carries cutting edge ideas on time travel and alien intervention. It shows two of the genre's masters at their groundbreaking best.