The €2m of rare books included incunabula, early books and manuscripts such as editions of Galileo Galilei and Isaac Newton.
A statement from the European Union's Judicial Cooperation Unit said the books had been “stolen by a highly sophisticated international organised crime group known to the authorities for committing a large number of burglaries throughout Europe”.
A joint investigation team was established with Europol which led to 45 arrest warrants issued at addresses across Europe with 13 of them in the UK.
As a result nine people were arrested in the UK and six in Romania.
The books had belonged to three dealers and were taken from trunks housed in a warehouse in Feltham, near Heathrow, on the night of January 29, 2017. The books had been destined for The California International Antiquarian Book Fair where the dealers had planned to offer them.
Only books were stolen during the break-in, despite other valuable items present in the warehouse.
The crime gang had abseiled into the warehouse to remove the books.
The Metropolitan Police, the Italian Carabinieri, the Romanian Directorate for Investigating Organized Crime and Terrorism (DIICOT) and the Romanian National Police all participated in the joint action.
The list of books stolen was published on the Stolen Book Database by the International League of Antiquarian Booksellers (ILAB) following the theft to alert dealers to potential stolen items circulating in the trade.