A cache of previously unknown letters to and from Benjamin Franklin has been found in an archive in the British Library. Professor Alan Houston found them while researching a book in 2007 (news travels slowly from the academic world):
What Houston had found was the handiwork of Thomas Birch, secretary to the Royal Society and a famously compulsive copyist of manuscripts. Birch had dined frequently with Franklin in London during the summer of 1757. Franklin by then was famous as a scientist for his experiments with electricity, but he wanted to show his British hosts that he was also politically important in the Colonies. Thus he carried with him, as a kind of calling card, a bound book of letters written by him, to him and about him during the Braddock affair. In his autobiography, Franklin referred to carrying a "Quire Book of Letters during this Transaction." The original quire book has never been found, but Birch copied it.
I'm fascinated by the detail that Franklin carried a quire of letters as a kind of press packet.
And this account of the find by Jennifer Howard in the Chronicle is much more detailed. Apparently Houton has kept his discovery secret for the last couple of years until he could publish the letters.