Baylor University Press has just made (2006) an English translation of Klauck's German work originally published in 1998, Die antike Briefliteratur und das Neue Testament. The English translation is not a straightforward translation of the earlier German work, but includes revisions and additions, especially in terms of explanatory notes on philological subjects, but also expansions and revisions of some arguments. This makes the book not only more use-friendly for the English-speaking student, but has made the book even better. So if you are only familiar with the German edition, you might want to check out the English one since it is really a second revised edition.
Klauck's book proves to me again how necessary, how urgent, it is for us to dismiss the canon boundaries that have locked us in the bible for too long. His book is quite comprehensive, covering letters in Early Judaism as well as the New Testament and beyond to the Greco-Roman world.
There are chapters on the "practical realities" of paper and the postal system in antiquity, nonliterary and diplomatic letters from the scores of documentary papyri available to us now, literary letters in poetry and philosophy, and the rhetoric of letters. His knowledge of the Greco-Roman world is extensive so he is able to include ancient letters that are little-known to most biblical scholars.
Each chapter is set up with students in mind, to function as a textbook as well as a reference resource. There are exercises at the end of the chapters, and an instructor's key at the end of the book. Each chapter contains valuable bibliographical sections that have been updated to 2005.