Hundreds of internationally important and irreplaceable books worth more than £2.5m that were stolen in a daring heist by abseiling burglars have been returned to their rightful owners.

Metropolitan police announced the successful conclusion on Tuesday of a near four-year police operation investigating the Mission Impossible-style theft of books that included rare works by Sir Isaac Newton, Galileo and the 18th-century Spanish painter Francisco de Goya.

DI Andy Durham, who led the investigation, said seeing the reaction of each victim on being reunited with their books had been an emotional experience. One man, Alessandro Bisello, “was so happy and said with great gusto: ‘Tonight we drink like lions!’. This made my day, seeing his reaction and joy.”

The books were stolen from a warehouse in west London in January 2017, where they were being stored on the way to a book fair in the US. Burglars cut holes in the roof before abseiling down to avoid sensors that would have set off alarms. Over five hours, they winched their haul up in 16 large bags. The books were later smuggled to Romania by organised criminals.

The concrete pit in Romania where the stolen books were found. Photograph: Met police

The books were eventually tracked down to Neamț, in north-east Romania, where they were discovered stacked in neatly wrapped packages in a concrete pit. Experts from the National Library of Romania helped with the investigation, storage and safe return of the books.

The Met said 83 of the books had some damage, mainly caused by water and mould owing to being hidden underground. Some spines were also broken.

The emotional return of 240 books, to four of the five victims, took place in Bucharest last month. Durham said: “It was lovely to see the joy of each victim being reunited with these irreplaceable books.”

Bisello, aged 30, told the Guardian that retrieving his 65 stolen books was like “finding a part of me that had crumbled or been stolen”.

“It’s because you spend so much time searching and collecting the books – it’s not just a two-month job – and you choose the books because you like them, not only for their value.”

Bisello, who heads up Bado e Mart auctions, a family business in Padova, recalled travelling to London after being told of the theft. “I was panicked, I was sad and very angry with what happened as all dealers who take part in international book fairs, send their best works. From what I know, the thieves rented a truck and they took the books from London to a country house in Romania. They dug a hole in the garage of this house and they put the books in luggage and covered the hole with cement.

”It’s like a loss – it’s like years of yours that were stolen and finally you get back what is rightfully yours. After three years we never believed we would get them back, I had lost all hope. We had little information about the investigation and we had to be very patient.

“Of course, when they told me [in September] that the books had been recovered I couldn’t believe it – it was like a dream come true – years of work finally came back to me.”

Bisello confirmed that the family did indeed drink “like lions!”.

Another of the victims, Alessandro Riquier, expressed his gratitude to the agencies involved in the case: “After three and a half years, finally this terrible story has a very happy ending.

A composite image showing a selection of the recovered stolen books. Photograph: Met police

“I went to Bucharest full of hope but also a little bit scared about the damaged books. I was very excited and it was a great joy to handle my books again and to see that, apart from one missing, and four books with variable damage, all the books were in good condition.”

Twelve men were jailed at Kingston crown court last month for their role in crimes committed across the UK. The men were identified as being part of a Romanian organised crime group responsible for a string of high-value warehouse burglaries in the UK. Police said the gang had typically avoided prosecution because members were flown into the UK to commit specific offences, and flown out as soon as the crimes were completed.

Four of the books are still missing and police are keen to hear from anyone with information. They are an illuminated manuscript attributed to the circle of Attavante degli Attavanti, published in north Italy in 1480, valued at about £24,000; a photo album of pictures taken in China and published in 1920, worth about £1,500; a collection of pressed butterflies, worth about £4,000; and La Saggia Pazzia by Antonio Maria Spelta, published between 1606 and 1607 and valued at about £1,500.