The secret sale from Stralsund's library triggered an outcry when it became public.

Berlin - A valuable, 400-year-old book about astronomy that was dumped by a German public library in a secret and controversial sell-off was returned to the city of Stralsund.

Police are investigating the municipal archivist who sold nearly 6 000 antique volumes to a second-hand book dealer in 2012 to raise 95 000 euros to cover library running costs. Many of the books were priceless.

One, a compendium on astronomy by the German mathematician Johannes Kepler (1571-1630), showed up later in a New York auction house with a ticket price of 250 000 dollars.

The auctioneer, Jonathan Hill, agreed to return the Kepler book to Stralsund at the price he paid a middleman to obtain it, but the amount remains confidential by mutual agreement.

Hill handed the book over in Stralsund, a Unesco World Heritage Site on the Baltic coast.

The returned book consists of treatises by Kepler, including Harmonices Mundi Libri V, his 1619 account of planetary motion. The secret sale from Stralsund's library triggered an outcry when it became public.

The city managed to repurchase 5 300 of the books from the book dealer and repurchase 63 more on the open market, but the rest are still missing. - Sapa-dpa