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The Hessians : mercenaries from Hessen-Kassel in the American Revolution

Author: Rodney Atwood
Publisher: Cambridge, Eng. ; New York : Cambridge University Press, 1980.

Couldn't find these two in world cat - but others will have better search techniques

The Hessians and the Other German Auxiliaries of Great Britain in the Revolution, Edward J. Lowell

New Jersey Hessians: Truth and Lore in the American Revolution, Peter T. Lubrecht

You might also find articles in this: "Journal of the American Revolution" if you can locate an index, or if someone can help

I can do you a pdf of the 6 or so relevant pp in John Mollo, Uniforms of the American Revolution if you do not have it.
Very general, but some useful pointers to further reading

If you can get to the national archives, looks like this might be of interest re: recruitment

Colonel William Faucitt
http://discovery.nationalarchives.gov.uk/details/r/C5910391
http://discovery.nationalarchives.gov.uk/details/r/C5910390

Best wishes

A