Try google book search for titles of books on the Hessian Troops.

This is a very good book on the Hessians. The bibliography should list useful sources.

http://books.google.com/books?id=GejQdlQrD-kC&pg=PA39&dq=von+ditfurth+regiment&lr=#PPP1,M1

You can write or e-mail directly to the State Archives in Marburg for assistance.

http://www.staatsarchive.de/archive/bl07.html

Good hunting

Ron