Who is Corona Machemer from Letters of E. B. White and what is their importance?

Corona Machemer had been White's editor at Harper's and became a companion and de facto nurse to White in his 80s, after the death of Katherine. Corona helped fill in the gaps caused by his near-complete blindness, and traveled to Sarasota, Florida with him to help organize some of his work for publication. White made it clear in other letters that theirs was strictly a friendly—not romantic—relationship.