Karl Barth (1886-1968) was the most significant Protestant theologian of the 20th century, of comparable stature to Calvin and regarded as the most influential since Schleiermacher.
Boston Collaborative Encyclopedia of Western Theology
...Karl
Barth on Revelation and God's Relationship to the World (Paul Cassell, 2005)
...The
Theology of Karl Barth (Derek Michaud, 2003)
...Karl
Barth (1886-1968) (Zdravko Kujundzija,
1999)
See also
George Hendry, Barth for Beginners. Theology Today, 1962.
Theology Today October 1986 Vol. 43, No. 3 (special issue on Barth)
John Webster, "Shapers of Protestantism: Karl Barth" in Alister E McGrath and Darren C Marks, The Blackwell Companion to Protestantism, 83-95.