I ran across a video documenting an interview where he talks about the book, and about his life. Around minute 30 he begins discussing the book itself, but just hearing about Gordon Parks is worthwhile watching the whole thing.
It's funny how your subject can choose you, in a way. Gordon Parks was a multidimensional, infinitely talented and determined person who once wrote a piano sonata (with no musical training) because he wished it. Many aspects of his life seem to mirror or complement those found in Turner himself.
From youtube site (about Gordon Parks):
"Gordon Roger Alexander Buchannan Parks (November 30, 1912 -- March 7, 2006) was a groundbreaking African-American photographer, musician, poet, novelist, journalist, activist and film director. He is best remembered for his photo essays for Life magazine and as the director of the 1971 film Shaft. .......Parks is remembered for his activism, filmmaking, photography, and writings. He was the first African American to work at Life magazine, and the first to write, direct, and score a Hollywood film.
Parks was a co-founder of Essence magazine and one of the early contributors to the blaxploitation genre. In 1984 Parks received an honorary Doctor of Humanities degree from Thiel College, a private, liberal arts college in Greenville, Pennsylvania. In 1989, the United States Library of Congress deemed The Learning Tree "culturally significant" due to its being the first major studio feature film directed by an African-American. Thus, the film was preserved in the United States National Film Registry. In 1995, Parks announced that he will donate his papers and entire artistic collection to the Library of Congress. One year later, "The Gordon Parks Collection" was currated. In 1997, the Corcoran Gallery of Art in Washington, D.C. mounted a career retrospective on Parks, Half Past Autumn: The Art of Gordon Parks. In 2000, the Library of Congress deemed Shaft to be "culturally significant", selecting it for NFR preservation as well.Parks himself said that freedom was the theme of all of his work, Not allowing anyone to set boundaries, cutting loose the imagination and then making the new horizons."