Beatrix Potter
Beatrix Potter, was a famous English author (and illustrator) of the beloved Peter Rabbit stories. Interestingly, Potter also wrote a secret diary between the ages of 15 and 30. This diary was handwritten (a difficulty in itself to decipher) and Potter went to the extra trouble of also writing it in code. This code was broken, and you can now purchase excerpts from the decoded version Beatrix Potter's Diary.
Not all diaries can be sold on Ebay for $40. The diary of Beatrix Potter would fetch a handsome price, as Beatrix Potter memorabilia regularly sells for several thousand pounds for even a single letter! Thankfully the diary is kept safely at the Victoria and Albert Museum in London, England.