In the early 1800s, the US was relatively new (25 years old), and so had not developed into an industrial economy. Cotton farming was widespread during the period as the conditions in the south permitted it, it was vital for the industrial revolution in Europe and was easily grown and cultivated.
So basically Cotton farming is your answer :)
For Copernicus, I think it was On the Revolution of the Celestial Spheres. I think For Newton it was Philosophiæ Naturalis Principia Mathematica.
Because of there natural resources like iron and coal.
