Funny you mentioned Suzy Bogguss -- one of my favorites. I LOVE her American Folk Songbook.
A few other favorite female singers: Tish Hinajosa, Judy Collins, Nanci Griffith, Madelaine Peyroux, Norah Jones, Melody Gardot, Diana Krall, Joan Baez, Joni Mitchell, Astrud Gilberto, Isabel Gilberto de Oliveira (known as Bebel Gilberto and Astrud's daughter), Petula Clark, Linda Ronstadt, Dusty Springfield, plus many others.