John Longhi interviewed Robert Crumb at the Last Gasp warehouse in San Francisco. He talks about the time he took a dose of bad LSD that negatively affected his brain for months (maybe for life, he thinks). Even though it was a bad experience, his creativity went into overdrive.
Crumb also recommends the book, The Cotton Kingdom : a traveller's observations on cotton and slavery in the American slave states, by Frederick Law Olmsted. It's available as an ebook at Archive.org.