Propping a wooden birdhouse filled with books on the back of his bike, Yan Defeng cycles around Zhengzhou in central China's Henan province, eager to find a home for his fourth birdhouse library.

The 36-year-old, who is self-employed, is spreading his joy of reading by distributing the birdhouses. It's part of a book exchange program inspired by something similar in the US.

The idea quickly migrated to Chinese cities. Participants take a free book from the birdhouse and replace it with one of their own.