Each year, 200 million workers from China's vast rural interior travel between cities and provinces in search of employment: the largest human migration in history. This indispensable army of labour accounts for half of China's GDP, but is an unorganized workforce--'scattered sand', in Chinese parlance--and the most marginalized and impoverished group of workers in the country.

For two years, the award-winning journalist Hsiao-Hung Pai traveled across China visiting labourers on Olympic construction sites, in the coal mines and brick kilns of the Yellow River region, and at the factories...