There is an article at Slate about Duke Ellington's underappreciated later work. I don't know enough about jazz to really add anything to this, so I'll just quote some of the article:
"He produced music that would not only extend the reaches of jazz but would become one of the largest and most original bodies of American music ever created. Ellington's early classics, produced between 1927 and 1940, have been often and rightly praised; his late work has been largely neglected. But the late work offers plenty of masterworks for the listener of sufficiently refined taste, or the one willing to sophisticate his or her taste. Put simply, Ellington's late work is largely a secret treasure. Anyone purporting to be civilized, or who desires to be, should have as many late Ellington recordings as possible in his or her audio collection.
"In conventional jazz writing, Ellington is said to have reached his musical peak in the three years of 1940 to 1942, when there is supposed to have been an unimpeachable balance between composition and personnel, resulting in stellar renditions and eloquent improvisation. But Ellington's ongoing evolution, from 1943 to the end of his life, runs counter to the standard critical take. In that last 30 years of band-leading and composing, Ellington achieved a remarkable range and authority. This was the result of both the time he had spent in the musical game and the vast technical and human experience his players were able to bring to the music, resulting in an abundance of varied tonal depth, emotional expansion, and subtlety—all of which is revealed in an increasing number of reissues and remixes now available. These recordings, stretching from the '40s to the '70s, demonstrate just how brilliantly Ellington and his band developed, decade by decade, almost right up to his death in 1974."