More autobiographies than biographies:
I have read Barbara Dickson's autobiography- very interesting, especially with the recording of all aspects of her career, from the Scottish folk scene in the 60s revival to her time in big musicals in London, and coming back to her folk roots.
Also Ralph McTell's "As far as I can Tell" which includes his childhood days in wartime and after, but it only covers up to the time he decided to go professional: a sequel is a must! (I even told him so after one of his concerts!)
Absolutely fascinating read: "From Fuzz to Folk" - autobiography of Ian Green of Greentrax recordings from his service in the Korean War , his time as a Police officer, and the "Fuzzfolk" folk club in Edinburgh, through to setting up his own recording and distribution business, Green tax recordings which has served so many Scottish artistes so well.
"The Queen Amang the Heather" - Sheila Stewart's book about her mother, Belle Steart, and such an insight into the life of the traveling people in Scotland. (Also read "Yellow on the Broom" and Red Rowans" by Betsy Whyte and anything by Jess Smith). Adam McNaughtan's song "Yellow on the Broom" was inspired by Betsy's book.
And not quite so folky, but Pam Ayres' autobiography (tho she does appear at folk festivals!), and Evelyn Glennie's "Good Vibrations".