Many were exported during cultural exchanges over centuries, and during social upheavals and wars in the 19th and 20th centuries, uncountable books were also taken away.

He notes that Kyoto's Buddhist temples have copies of the Chinese Buddhist Canon published in the Song Dynasty (960-1279), while it is difficult to find that complete set in China.

According to Zhang, it's most important to share the digital resources with all sides than to focus on the return of such volumes to China.

NLC has a 2.6-million-volume collection of ancient Chinese books. It began a long project to digitalize them in 2012, and shares the database with overseas counterparts. Zhang expects fellow institutions to reciprocate.

In the newest NLC-CEAL agreement, more than 10,000 kinds of fine ancient Chinese book collections all across North America will be digitalized for wider study.

Zhang believes that the Chinese communities overseas are also holders of many lost ancient books and unique historical files.

"We are running the national library for all Chinese people in the world, so our definition of ancient books cannot be too narrow. Our compatriots living abroad for centuries can also join in our common work by providing their recorded history."

Zhang says the library will seek help from Chinese embassies and consulates in that effort.

"However, the most pitiful so far is our communications with Japan, where there is the largest overseas reservoir of ancient Chinese books, mainly in the hands of private collectors rather than state-level institutions.

"As we try to warm up the cultural connection between two sides, something may gradually change."