In the first part of the sixth century, variant forms of Monophysitism existed. In 'Christology after Chalcedon', Iain Torrance provides a theological introduction and a translation of the letters between Severus of Antioch and Sergius the Grammarian. Severus was the Monophysite Patriarch of Antioch - a leader of the moderate Monophysites whose doctrine adhered more closely to Catholic teaching and whose primary divergence from orthodoxy was terminological. Though little is known of Sergius, it is apparent from his letters that he was a Monophysite of the more extreme sort. The correspondence between Sergius and Severus comprises three letters from Sergius, three replies by Severus, and an apology by Sergius.
This was a hard book for me to read. Iain Torrance IS a good communicator, but this subject is replete with a very rough theological terrain. Torrance helps point out some of the "flaws" in both East and West, and for that I found it to be helpful. That said, it would not be the end of the world if one did not read this volume, you can get most of what he says elsewhere (especially in TF Torrance). If you do, you will benefit from it... it is NOT a waste of time at all.