"Celestial Matters" by Richard Garfinkle
For the last nine-hundred years, the Delian League has been at war with it's mortal enemy, the "Middle Kingdom" (or "China," if you prefer), ever since Alexander took one of their provinces. Between the two, they have basically divided the Earth (the Americas, "Atanteas", are split east/west, and a constant battleground, for example).Now, however, the Delian League feels it's in danger of losing this endless war, and has set up several research projects to create new weapons.
One of these is Project Sunthief. The books narrator, Aias, is the head of this project which plans to fly the large Celestial Ship "Chandra's Tear" to the Sun, scoop out a chunk of Helios-matter, and use it to destroy the "Middler" kingdom's capital, Hang Chow.
A basically simple project is complicated by political moves, a spy on board who might be Aias's friend, Middler attacks, Gods who may or may not be on their side, oh, and the slight possibility that after they net some of the sun, the ship will be destroyed...
Every so often you come across an Alternate History book that kinda stretches the boundaries of what you consider A.H. One such is Celestial Matters, by Richard Garfinkle. The basic split point here is that Alexander went to study in Sparta, then formed an "alliance" as such with Aristotle, to create the weapons with which to run his campaigns.
The "Delian League" has lasted for a thousand years.
But while this is the historical split, there's a much greater physical one. For on this Earth, Aristotelian science really is true. Planets really do move in crystal spheres about an unmoving Earth beneath a vast shell of fixed stars, and they really are made out of different stuff from mere "Earthly" matter. Projectiles actually do travel in straight lines until they stop, and you really can cure someone by balancing his humors.
"Celestial Ships" are built of matter mined from the nearby planets – Selena, Hermes & Aphrodite – which because it is celestial matter, continues to "orbit" above Earth. They were first created just a few decades prior to counter the "Battle Kites" of the Middlers – which "fly" the Xi currents (Chinese science is Tao-istic).
"Space" travel makes up much of this book, and is quite unusual. "Space" is filled with air (though it gets "purer" as one goes higher – which can cause problems) and navigation is a matter of "flying" to a planet, slipping underneath the edge of the Crystal Sphere, and flying to the next one. Planets are quite small in this universe. While not stated, I doubt that any of the nearby ones are larger than a hundred miles across.
It's a very strange world. Interesting, but strange. Every so often the characters say/do something that is completely, utterly, weird – yet makes sense within the world of Aristotelian physics.
The basic story is interesting too. It's a good read, I reccommend it.