The discovery has revealed how British troops occupied the dictator's family home to guard against his return during the latter part of World War Two.
Shortly before Mussolini was shot by the resistance, Captain Beck flew to the Villa Carpena in Predappio near Rimini every other day for a fortnight. British troops had occupied the villa and were guarding against the dictator's return to where he was born and later buried.
It appears that Captain Beck liberated a few items of memorabilia during his visits and even posed for a photograph in the dictator's "mutz," his tasselled black hat.
After his death last year, his widow sent the collection to be sold. Scarborough auctioneers, H.C.Chapman, spent several weeks cataloguing the haul.
Captain Beck's trophies included government telegrams, letters to Mussolini and unpublished photographs of him with mistress, Clara Petacci.
There was also a first edition copy of Mein Kampf, schoolbooks that belonged to Mussolini's children and newspaper cuttings from the 1920s and 1930s. These included a report in the Daily Express in which the dictator announced that Julius Caesar was his master.
The auction also included Captain Beck's RAF flying log with entries referring to his duty at the villa.
Terry Charman, an Imperial War Museum historian said: "Historically the collection isn't interesting because most of the articles date from the early years of the fascist regime."
However, there were many people who were interested in such items. "The late Marquess of Bath collected everything to do with Hitler, Churchill and the Duke of Windsor. That type of collector would be interested ... rather than an institution," Mr Charman said.
More people collected Hitler than Mussolini, he added. "If this was a collection from Hitler's library, there would have been even more interest. But Mussolini's books do indicate his wide-ranging cultural interests. He wasn't the ignorant thug that people think." Mr Charman said he could not recall much Mussolini memorabilia coming on to the market.
The villa at Predappio was the dictator's family home, but after he became head of government in 1922, Mussolini spent much of his time in Rome, which became his permanent base in 1929.
Mr Chapman said the items from Villa Carpena shed little light on Mussolini's exploits in Abyssinia, Spain and Albania or on Italy's entry into World War Two.