When Liu Tao, a writer and photographer in North China's Shanxi province, perused a secondhand book fair one weekend in 2015, a bundle of 12 small notebooks caught his eye.

"I collect old items, not to lock them in dusty cupboards, but to research and learn the stories behind them," Liu wrote later in memory of that day. "I was rather interested in these little notebooks because they were the daily financial records of a family across a time span of 41 years, from July 1952 to May 1993."

When the shop owner named a price as high as 800 yuan ($111), almost 20 times the price of new notebooks of the same size and quality, Liu didn't bat an eyelid and bought them.

"I liked them and I knew their value," he said. "I only regret not having purchased more materials because I had to spend more time finding the details of that time in history later for my research."

From the 12 notebooks, Liu was able to glean intimate details about the daily transactions made over a 41-year period in the daily life of Zhang Jianmin, a Party official in Shanxi.

What he discovered was so fascinating that Liu put it all into a book titled The Financial Records of Jianmin: 1952-1993, The Digital Life of an Old Communist Party of China Member, which was published in August last year.