I am always impressed by the detail of Thomas Jefferson’s records that he kept. From schedules and amounts of what he planted and harvested, to details of plantation operation, to notes about his travels and worldly accomplishments. He is by far one of the most personally well-documented figures in American history.
One amazing thing he used was the Polygraph. It is essentially a manual “copy machine” where writing with a pen on side would be mechanically connected to a pen on the other side. Whatever was written by hand would be copied by pen to paper by the second pen. He started using this in 1804, and more details and an image can be found here: