Nick Ashford

Nick Ashford, along with his wife Valerie Simpson, didn't just write great songs for Diana Ross, Marvin & Tammi, Gladys Knight, Chaka Khan and others, they also made some great R&B albums as a duo.
Their best albums played like scenes from a marriage.  It was a tight songwriting partnership.  The endurance of life and romance always bubbling beneath the melodies.
Both are very underrated as singers.  Go find their albums, preferably late 70's ones.  All still in print.