If you ever meet Robert Crumb, don't tell him to Keep on Truckin'; it's a bit of a sore spot. The best way to find out about the celebrated American illustrator is to hear it from the man himself.
The first video is an excerpt from the excellent 1994 documentary "Crumb." Second is a 2011 interview. Finally the third video is an excerpt of a BBC documentary from 1987.
Official Website: http://www.crumbproducts.com/