This really does sound like an outstanding blog:
Letters of Note is an attempt to gather and sort fascinating letters, postcards, telegrams, faxes, and memos. Scans/photos where possible. Fakes will be sneered at. Updated as often as possible; usually each weekday.
Thank you @davidkfiction for tweeting a letter written by former slave Jourdon Anderson to his previous master who was begging him to return to work:
Say howdy to George Carter, and thank him for taking the pistol from you when you were shooting at me.
You can read the full letter here, and find out more about the blog here. They also release collections in book format.