Three books by daughter Helga Sandburg:
Sweet Music, A Book of Family Reminiscence and Song, 1963
A Great and Glorious Romance: The Story of Carl Sandburg and Lilian Steichen
Where Love Begins: A Portrait of Carl Sandburg, His Family As Seen Through the Eyes of His Youngest Daughter
Some biographical information and family history. Have only read the first but reviews indicate they focus mainly on the family and only peripherally on the development of The American Songbag and the New American Songbag.