“Thank you for all the years of understanding each other,” Carly Simon wrote in my copy of Boys in the Trees. We were sitting together on a couch, just the two of us perched high above the East River. It was a cool evening in the fall of 2016 and I’d just finished an interview with Carly about her memoir. Her sister Joanna (“Joey”) invited me to conduct the interview at her home. I arrived with a…