The name Guglielmo Libri will mean little to anyone outside the inner circles of academia. But a mere mention of the 19th-century Tuscan noble and polymath to European scholars still has the power to provoke hand-wringing and despair.

Count Guglielmo Libri Carucci dalla Sommaja was more than a respected scientist and a decorated professor of mathematics. He was also – and more notoriously – a book thief, guilty of intellectual larceny on an international scale.

In the mid-1800s, Libri pilfered tens of thousands of precious manuscripts, tomes and documents from Italian and French libraries, including 72 letters written by the great French philosopher and mathematician René Descartes.

Now, in an emotional ceremony, one of the letters has been handed back to France after collecting dust in a library at a small American college since 1902.

The letter, described as "a wonderful discovery for science", is dated 27 May 1641 and concerns the publication of Descartes's treatise, Meditations on First Philosophy – subtitled In Which the Existence of God and the Immortality of the Soul Are Demonstrated – that year. It was written to Father Marin Mersenne, who was overseeing the book's publication.

Academics had known of the letter's existence for more than 300 years but not its contents as nobody, apart from a Haverford College undergraduate, had examined it. As scholars pore over the contents, its discovery has once more put the infamous Libri under the spotlight.

Born 1 January 1803 in Florence, he was a precocious academic who, at the age of 20, was appointed professor of mathematical physics at Pisa, and had a fascination with ancient books and manuscripts.

Threatened with arrest for his political activities, he fled to France, where he was elected to the French Academy of Sciences and awarded the Légion d'Honneur. His love and knowledge of books were recognised when he was appointed Inspector of Libraries, tasked with cataloguing valuable works. Instead of documenting them, however, he began stealing them.

Tipped off about his imminent arrest, Libri fled once more – to England, bringing with him around 30,000 books and manuscripts in 18 large trunks, including works by Galileo and Copernicus. Although found guilty of theft by a French court and sentenced in absentia to 10 years' in jail in 1850, Libri enjoyed the high life in London, funded by selling the stolen tomes.

He returned to Italy to die in 1868. Learning of his death, the French government requested the return of some of the manuscripts and offered to buy back those that had been sold. Some were returned, but tens of thousands of other precious stolen works simply disappeared.

The Descartes letter had been donated to Haverford, near Philadelphia, by the widow of an alumnus and remained in its library, unnoticed, until a philosophy scholar at Utrecht University in the Netherlands stumbled across a reference to it on the internet. He contacted the college, which immediately offered to return it to France. The French Institute plans to publish it in a collection later this year.