Augustine used the word "tractate" for any prepared spoken communication. Many of these tractates he delivered as homilies or sermons from the pulpit, and they were copied by stenographers in the congregation; others he dictated directly to stenographers in private, but in a way that they could later be read aloud in a church as a homily by others.
All of these tractates were certainly written over the period of the years between 408 and 420. there is agreement that tractates 1-16 were preached by Augustine in the winter of 406-407, and tractates 55 to 124 covering John 13-31 were prepared in 419.
Above: A page of the Tractates on the Gospel of John (In Johannis Evangelium Tractatus) from the first half of the twelfth century. It is written in a well-formed large Carolingian script, with Romanesque white-vine decoration. Tractates 55 to 124 are different in style to the earlier tractates, and this has led to the conclusion that they were not spoken to a congregation, but probably to stenographers in private. Certainly, Augustine had long been pressed by people to complete his explanatory coverage of all chapters of the Gospel of John, and accomplished it in this way.
His commentary on the Gospel of John is primarily pastoral rather than consciously being highly theological. Augustine's focus in these tractates is on Incarnation, or the Word made flesh. This is not only a distinctive and important theme in the Gospel of John; it was a significant part of Augustine's own conversion, as mentioned in the Confessions (e.g. Book 7, chapter 9, lines 13-14).
Tractates Online. On this web site from the CCEL Fathers of the Church series is the Gibb-Innes translation of all 124 tractates into English. http://www.ccel.org/fathers/NPNF1-07/t.htm Also at: http://www.newadvent.org/fathers/1701.htm
For further reading
Augustine through the Ages: An Encyclopaedia. ISBN: 0-8028-3843-X Published in 1999, 880 pages. Edited by Allan Fitzgerald O.S.A. (Stocks of this publication are now exhausted, but second-hand copies are sometimes available for purchase online from Amazon.com.)
The encyclopaedia is the product of more than 140 leading scholars throughout the world. This comprehensive publication contains over 400 articles that cover every aspect of the life and writings of Augustine of Hippo (354-430). It traces his profound influence on the church and the development of Western thought through the past two millennia…." AN2146