What you quoted is not a direct source either. There are simply direct quotes included.
The letters that were summed up and paraphrased in the interview are all included in the book.
Reading the Man: A Portrait of Robert E. Lee Through His Private Letters https://www.amazon.com/dp/B000QCSA72/ref=cm_sw_r_cp_apa_-UwezbYKNWE53
Here is an article written by Elizabeth Brown Pryor with directs quotes as well.
https://opinionator.blogs.nytimes.com/2011/04/19/the-general-in-his-study/?_r=0