Deborah Solomon travels to Lafayette, La. to meet the artist’s sister and learn about his dyslexia and their fundamentalist Christian upbringing, talks to a classmate at Black Mountain College from whom he stole a quilt to use in an early artwork (“The next time I saw it was at the Leo Castelli Gallery”), and has coffee-with-Häagen-Dazs in a Williamsburg loft with Susan Weil, Rauschenberg’s ex-wife and the woman who taught him how to make photograms.