Joni M's middle period has been mentioned. Don't forget her early stuff. (Song for a Seagull, Joni Mitchell, Clouds, Ladies of the Canyon etc.) She was so far ahead of everyone else back then.
Joan Baez tends to split opinion, but I rate her. If the early stuff grates, try 'Diamonds and Rust' for a more mature lady and her take on Dylan, Jackson Browne etc.
Recent home grown:
Thea Gilmore Intelligent rocky/folky stuff.
Karine Polwart. Contemporary folkie stuff.
Kate Rusby. Fab voice but can be a bit samey.
'Mericun:
Beth Nielsen Chapman. Contemporary country/folkie stuff. If nothing else, check out 'Seven Shades of Blue' written for her late hubbie.
Dory Previn. Died last year aged 80+!. Early 'singer songwriter' stuff.
Judee Sill. Interesting character. Dead former drug addict/prostitute and sublime songwriter. Check out 'The Kiss', 'Jesus Was a Crossmaker', etc.
And although pretty much ubiquitous and now possibly over-hyped, two classics by Jennifer Warnes are essential purchases: 'Famous Blue Raincoat' and 'The Hunter.'
Mull