These are almost an art form in themselves.
My favs include
Bob Dylan The Bootleg Series-the late great John Bauldie essential booklet on Dylans 1991 triple CD of unreleased tracks...
Bob Dylan-World Gone Wrong-Dylan's own surreal take on the modern world and the origins of his second album of traditional songs in the 90's.
Steely Dan-their complete remastered catalogue-just about zilch on music but another insight into the world of Becker and Fagen-funny,sardonic,mad.
The Waterboys-Too Close To Heaven-honest insight by Mike Scott on his outtakes from 86 onwards-personally these made me both nostalgic and sad as they are a testament to how Scott lost the plot(musically)around this time.
There's loads others,The Who's recent remasters have decent notes and the booklets with the 6 CD Anthology Of American Folk Music (Harry Smith)are superb..