The publisher Peter Lang just distributed a catalogue with an overview of their impressive series called Linguistic Insights, Studies in language and communication, edited by Maurizio Gotti from the University of Bergamo.

The series contains several titles that are relevant for the study of the language of Late Modern English letters:

• Marina Dossena and Susan M. Fitzmaurice (eds.), Business and Official Correspondence: Historical Investigations (2006)
• Marina Dossena and Ingrid Tieken-Boon van Ostade (eds.), Studies in Late Modern English Correspondence (2008).

But apart from these there are many other titles that will of interest for this subject and that are therefore well worth reading.