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Now each has produced a slim stocking stuffer for the Christmas season, distilling that wisdom into an afternoon's read, complete with insight, jokes, song lyrics and a few entertaining nuggets from their remarkable lives.
If Nelson and Parton are talking, it's worth listening.
These two backwoods bodhisattvas have been contributing to the greater intelligence of the human race for more than a half-century each.
Unmotivated by trends, they have found themselves at the centre of the action multiple times.
After inventing ''outlaw country'', Willie Nelson anticipated the American song-book craze by about 20 years when he released Stardust in 1978, introducing a new demographic to the songs of the greatest generation.
A bona fide country star, Parton fused country and disco with 9 to 5, and wrote the Whitney Houston blockbuster I Will Always Love You, which has been a radio hit four times in three different decades.
If they never picked up a musical instrument or stood in front of a microphone, Parton and Nelson would both still be legendary as songwriters.
Yet both made themselves into quadruple threats, writing, playing, singing and performing in movies. They didn't quit there.
Parton gave an economic bone-marrow transplant to her Tennessee community with the amusement complex Dollywood, which became one of the region's largest employers.
Their books are different in style and tone.
Nelson's book, Roll Me Up and Smoke Me When I Die: Musings from the Road is a Rabelaisian idyll, mixing contemporary journal entries (from 2011 and 2012) with reminiscences from friends and family and plenty of profane good humour.
Parton's entry, Dream More: Celebrate the Dreamer in You, grew out of her University of Tennessee commencement address in 2009 (a ceremony during which she received an honorary degree and became Dr Dolly).
Perhaps more self-consciously inspirational, it is also packed with humour, which never violates a PG rating.