(CNN) -- A rare letter written by Napoleon Bonaparte in English will go to the auction block in France on Sunday.
The letter could fetch close to $100,000 in the auction in the French town of Fontainebleau, south of Paris.
Auction house Osenat describes it as the first letter the French emperor wrote in English on St. Helena, after he was defeated and exiled to the British island to live under military guard.
Learning English behind the backs of his captors "was a sort of revenge, a historical revenge" for Napoleon, said Jean-Christophe Chataigner of Osenat.
"He was imprisoned by the English... and he wants to continue to have a certain degree of independence, of freedom, and to be able to learn English without his jailers knowing it was a great motivation for him," Chataigner said.
The auctioneer said Napoleon picked up English relatively quickly and well.
"I think that French people who learn English today make lots more mistakes than Napoleon at the time, so it's a letter which is relatively well-written," he said. "There are very few mistakes apart from a few words which have two meanings in English."
Napoleon learned the language in two or three years, Chataigner said.
"He's really a very good student, although he was a good student in lots of fields so it's maybe not that surprising," he added.