In 2017, a Belgian woman named Tania Grégoire sent a trove of 150 antique books to Sotheby’s auction house in London. Experts immediately smelled something fishy about the collection. Many of the title pages and bindings, which is where a library stamp usually appears, were missing. In a few cases, library stamps had been scraped off or chemically removed. Some library and shelving marks, however, were still visible and they linked the books to Germany’s University of Bonn, Catherine Hickley at The Art Newspaper reports.

The university has an extensive catalogue of the 180,000 books that went missing during the war years. While most of the lost books were incinerated when the campus was firebombed, some disappeared in the years after the war when the area was occupied by Belgian troops. At that time, between the years 1946 and 1950, valuable volumes that remained from the library were stored in air raid shelters.

In a press release, Michael Herkenhoff, the university library’s curator of manuscripts and historic books, says it’s possible that Belgian soldiers plundered the book depots for valuable volumes.

That’s likely the case with Grégoire's books. Reporting from Berlin, Kate Connolly at The Guardian writes that Grégoire’s father was stationed in Bonn during the war. After learning that the books she was looking to auction may have been looted, she led authorities to 450 more volumes stored in her garage in Brussels, beginning the process of repatriation.

In total, the trove returned to the library consists of more than 600 volumes. Among them there is a 13th-century manuscript of the Comedies of Terence, a beautifully decorated prayer book from the 17th century, one of the first books printed using Greek letters and a large collection of bird books. The most valuable item, according to Antiques Trade Gazette, was a copy of John James Audubon’s The Birds of America (a copy sold for $10 million last year, reports Pickford).

For her cooperation, Grégoire received a finder’s fee from the German state. “She showed a moral approach to the matter from the start, having been quite shocked to find how the books had come into her possession, and was at pains to treat us fairly,” Herkenhoff tells Connolly.

The books aren’t the only ones that have been returned to the Bonn library in recent years. According to the press release, in 2011 an American soldier returned a book he’d taken from the library, and in 2018 the heirs of another U.S. soldier returned three books they found in his estate.

Books are flowing the other way as well. Milton Esterow at The New York Times reports that libraries in Germany and Austria, which received thousands of books stolen from private citizens, are still in the process of repatriating them, having returned 30,000 books to 600 owners over the last decade.