The River Chronicles

The sky is gray, the snow is gray. Du Jianghe, a scavenger youth from the lower district, digs out a pitch-black iron ruler from a garbage pit. The ruler is ice-cold to the touch, yet it feels like a long-lost family member returning to his hand. That night, his adoptive father dies mysteriously, clutching half a torn map in his hand—depicting a rusty iron door with nine concentric circles, each corresponding to a ruler. Du Jianghe doesn't know what lies behind that door, nor why people from the upper district are chasing him. He only knows: to find the answers, he must first collect all nine rulers. Nine rulers, nine journeys home, and at the end of the road stands the iron door that has been there for three hundred years. — Description from novelcodex.org