There's no tidy, agreed-upon divisions.
By "mid- and late-period" Ellington, I'm basically referring to everything after the Blanton-Webster band.
So, yes, mid-1940s onward.
My recommendations:
- The World of Duke Ellington by Stanley Dance
- Beyond Category: The Life and Genius of Duke Ellington by John Edward Hasse
- Duke: A Portrait of Duke Ellington by Derek Jewell
- Duke Ellington in Person: An Intimate Memoir by Mercer Ellington
I would avoid the biographies by James Lincoln Collier and Terry Teachout, who both fail to understand/recognize the brilliance and importance of mid- and late-period Ellington.