Hundreds of priceless manuscripts and documents believed to have been looted by Belgian soldiers from a German library at the end of the second world war were returned on Thursday.

The works, which were thought to have been irretrievably lost, included rare medieval manuscripts, early 15th-century prints, historical maps and the 19th-century illustrated bird books from the library of the celebrated German ornithologist and explorer Maximilian of Wied-Neuwied.

Michael Herkenhoff, the curator of manuscripts and old books at the University and Regional Library of Bonn (ULB), described the return of the works as “a huge surprise”. It is the largest return of lost books in the library’s 200-year history, as well as one of the largest single returns of cultural items lost in the war.

“We need to clean many of the works, and to repair and catalogue them anew, but this is a luxury challenge to have because we really never thought we would see these volumes again,” Herkenhoff said.

Drawings from John James Audubon’s The Birds of America, from the 1840s. Photograph: Universitaets- und Landesbibiliothek Bonn/Bonn University

The 600 works came to light after they were offered for sale by Tania Grégoire, a Belgian. Her book-loving father was stationed in Bonn as a soldier during the postwar occupation and, although the circumstances remain unclear, is believed to have been involved in bringing the books to Belgium. Grégoire offered the items to the London branch of the auction house Sotheby’s in autumn 2017.

“Our team of specialists recognised immediately when it started researching the collection that there was something dodgy about it, because there were books without bindings, torn-out title pages and expunged library stamps, in an obvious attempt in the past to conceal their origins,” said Lukas Baumann, the cataloguing assistant in Sotheby’s books department who helped uncover the collection’s provenance.

“They discovered library shelf marks and signatures that hadn’t been removed, which was how we were able to establish that they belonged to ULB.”

Contacted by Sotheby’s, the library provided a comprehensive inventory of lost objects, which most cultural institutions compiled after the second world war, from which a match was made.

The books, which amount to the largest return of objects in the history of Sotheby’s, according to Baumann, are due to be returned to the library in the presence of historians and cultural figureheads. Grégoire, who received an undisclosed compensatory payment, said to be considerably lower than the books’ value, from the cultural foundation of the German states in recognition of her willingness to return the books to their rightful owner, will also be present.

A richly-decorated manuscript. Photograph: OS12002 #58517/Bonn University

“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,” said Herkenhoff.

“We had an improbable amount of luck,” he added. “First of all, it’s not all that usual in our experience that an auction house contacts you to let you know they think they’ve found something of yours, and secondly, in Madame Grégoire we had someone with whom we could have a proper dialogue.”

Monika Grütters, the German cultural minister, described the case as an “exemplary resolution” to one of many second world war restitution cases, most of which are far less straightforward.

The ULB case is unusual, involving as it does the return of looted objects to Germany. Far more typical is the return of objects to individuals or countries that were looted by Nazi Germany. During the Nazi dictatorship millions of objects were expropriated mainly from Jewish families and other regime opponents across the Third Reich.

Baumann said Sotheby’s always sought to clarify the provenance of objects it was asked to sell, and to facilitate their return where appropriate.

“This is a perfect example of how fruitful public market and institution collaboration can be,” he said.

He said out of 15,000 object offered to Sotheby’s for sale across Europe every year, only about 50 had a question mark over their origins, and of those about 30 proved “difficult cases to solve”.

Medievalists across Europe, said to be keen to view the returned works, will be able to do so from Thursday. “The works include unique manuscripts which have not been viewed for over 70 years,” Herkenhoff said.

“There is great excitement amongst academics. The whole thing has been very emotional.”

The ULB evacuated its contents to underground storage facilities in 1943. They were returned to Bonn in 1946, following the end of the war, but stored in former air raid shelters because the library had been destroyed by bombing. Only when the books were returned to a newly built library in 1951 did librarians become aware of the extent of the gaping hole in their collections, amounting to a loss of around 180,000 volumes. Many were lost when bombing destroyed the building, but a large number also disappeared at the end of the war and in the immediate aftermath and were assumed destroyed, lost or stolen.

Four other books were returned to the library in recent years, including by an American soldier who brought back a 16th-century manuscript in person in 2011, and last year, when the heirs of another soldier turned up offering three books from the 16th and 17th centuries which he had taken as war booty.