The Six Directions of Space by Alastair Reynolds. This is a short book -- reading its 85 pages this morning was akin to watching a 90-minute movie -- so this will be a short review.
I am a sucker for alternate histories, and this story has the Mongol Empire of Genghis Khan not only taking over the entire world, but also maintaining control for a thousand years and discovering interstellar travel thanks to the relics of a long-gone and unimaginably alien race. All this provides a great canvas for a tale of brutal politics, complex spywork, parallel worlds, space battles, and ponies. It sometimes feels like a short story aching to be a novel, but it has enough rewards in its own right to be worth the read.
It won't take you long, anyway.