Karl Marx's letter to Abraham Lincoln (1864)
marxists.org
marxists.org
Unfortunately, after Lincoln's assassination, this battle was lost in the years after.
Disclaimer: I'm no fan of Marx.
I highly recommend David McCullough's biography "John Adams." Even if you're not really into history (I'm not) it's actually a pretty good read and entertainingly covers most of US History from 1750 to 1830.
This does it for the history buff in me - startup or not. Anyway Lincoln was a serial entrepreneur.
That said, I think its counterproducent for people who love to claim that are trying to change the world, underestimate the significance of history. And this letter is, at least for me, a unique, yet not well known, piece for understanding history.