Bruce E. Burgoyne had done some archival research on the Germans, he published a lot of books (more than 35) through the Heritage Books, such as:

A Hessian Officer's Diary of the American Revolution Translated From An Anonymous Ansbach-Bayreuth Diary and The Prechtel Diary
Diaries of Two Ansbach Jaegers
Enemy Views: The American Revolutionary War as Recorded by the Hessian Participants
Hessian Chaplains: Their Diaries and Duties
Journal of a Hessian Grenadier Battalion
Most Illustrious Hereditary Prince: Letters from Members of the Hesse-Hanau Military Contingent in Service of England During the American Revolutionary War
Notes from the British Museum
Revolutionary War Letters Written by Hessian Officers
The Hesse-Cassel Mirbach Regiment in the American Revolution
The 3rd English-Waldeck Regiment in the American Revolutionary War
These Were the Hessians
.......

I only have one copy of 'Notes from the British Museum (Re: The Heissians)', a solid book with many details.
PS: Johann Conrad Döhla's 'A Hessian Diary of the American Revolution' was also translated by Bruce E. Burgoyne, this book was published by University of Oklahoma Press.