In the early 1970's, Gram Parsons and Emmylou Harris collaborated in such a remarkable way that Parsons' untimely death in 1973 left both enormous unexplored possibilities, but also Emmylou trying to find a way to carry on. To this day, she has done so brilliantly.
On Gram Parsons and his influence on Emmylou Harris:
Album recommendation: Grievous Angel (Gram Parsons with Emmylou Harris (original cover credit changed upon posthumous release of Parsons' album), 1974). For an earlier post on Parsons, see here; on Harris, see here and here.