The Tangled Lands by Paolo Bacigalupi and Tobias S. Buckell
Khaim, the last city of a once-great empire, clings to life.
This is a world poisoned by its addiction to magic. Every time a spell is cast, toxic bramble sprouts, bursting forth from tilled fields, thrusting up from between cobblestones. A bit of magic, and bramble follows until whole cities are consumed by tangling vines. Armies hack and burn… yet in a world that can’t give up magic’s sweet succour, the bramble can’t be stopped.
When Jeoz the alchemist discovers a potent new weapon to wield against the bramble, his invention promises Khaim freedom from fear and starvation. But these are twisted times and the bramble is not the only corruption that blights the land.