“We have a small team of researchers and, since we started 10 years ago, we have returned 900 books to 20 countries,” Mr. Finsterwalder, the researcher, said.

“Thousands of books were marked by the Nazis with the letter J, an abbreviation for Judenbücher — Jewish books,” he said. “These were erased after the war and replaced with the letter G, as in Geschenk — gifts.”

Overall, libraries in Germany have returned about 15,000 books since 2005, Maria Kesting, a provenance researcher at the Hamburg State and University Library, said. “I have returned books to about 360 heirs, owners and institutions in the United States, Britain, Germany, Israel, South Africa, France and other countries,” she said.

Researchers say the process can be complicated because libraries across a country like Germany will often lack a central database of their holdings or the money to do more than minimal provenance research. Since 2008, the German Lost Art Foundation, which is funded by the federal government there, has provided $5.6 million for provenance research on books “and related items” in German libraries. The foundation publishes descriptions of books with photos in its database when owners or their heirs cannot be located.

Wesley Fisher, research director of the Claims Conference, said that it and the World Jewish Restitution Organization have helped train 180 provenance researchers in Germany, Lithuania, Greece, Italy and Croatia.

In the absence of a coordinated government effort, researchers from libraries in nine German cities have organized to trade notes, according to Ms. Kesting, who said they meet twice a year. Among the problems they have found is that the Gestapo often distributed looted works to multiple libraries.