Since 2005, when the program to collect family letters during the war was initiated, about 200 letters have been collected from around China. But there are still more with many elderly Chinese that need to be collected and archived.

"So we still have a lot of work to do," Zhang says. "This is just a start.

"Publishers from both the mainland and Taiwan have published some letters by military officials. So in our program we emphasize more the letters from civilians."

Although the letters are personal, if the writers were involved in major historical events, then the letters can form part of the evidence, especially in disputed cases.

For example, the book includes two letters by Liu Zongxin, then a graduate student at Tongji University's school of medicine.

In his last letter, written on Dec 26, 1941, he said he was treating patients infected by an epidemic in Zhejiang province.

Four days later, he died of the same infection in Yiwu.

The letters testify that the Japanese army used bacteria not only in Northeast China, but also in coastal provinces such as Zhejiang.

The letters also show how Chinese sacrificed their lives to keep the country safe.

"We can feel that strong spirit of our nation in the letters," Zhang says.

"It's very touching and infectiously powerful."