Primary sources are primary sources, but I have always loved seeing the original handwritten/typed document. I'm no handwriting analyst, but I think everyone can at least get somewhat of a sense of how the author is feeling at that particular moment. When there are revisions or scribbles on the document, even better—readers can see the author's ideas develop.
The best part of these correspondence documents, however, are that they are intended for only one reader. A handwritten letter from Jimi Hendrix to his girlfriend is as hilarious as you would imagine.
Of all the Beatles-related books I have read, my favorite would probably be George Harrison's "I Me Mine," in large part because it included his handwritten lyrics on napkins and random letterheads. I'm not sure if they were recreated to look original – I'm choosing to believe they weren't – but it helped put me at the scene of the crime.
So in tribute, sort of, I tracked down a nice original document from Franklin Delano Roosevelt. Below is the first draft – including scribbles – of FDR's "Infamy" speech after the bombings at Pearl Harbor.