Surprised no one mentioned Lime Rock Park: The Secret Valley of Racing. Great independant documentary about the history of Lime Rock. It really captures the magic of LRP.
More info here:
Chris Szwedo Productions
I invited Chris Szwedo to show his "Gullwing at Twilight" documentary at my PCA region's monthly meeting a few years back. Great down to earth guy - very passionate about his work, and a great artist as well.