An excellent book on this topic by an academic is James Weingartner's
Crossroads of Death: The Story of the Malmedy Massacre and Trial. Weingartner's book details all aspects of the event and is particularly excellent in dealing with the trail and post trail implications of the investigation. His chapter on McCarthy's involvement is fascinating for those unfamiliar with his role. If you have interest in this topic and have not read the book, I highly recommend it. Weingartner also authored the first academic treatment of LSAH titled
Hitler’s Guard. It is definitely a dated book, given all the information now available, but also an excellent read if you can find a copy.
A general comment about shootings of unarmed POWs: Gerald Astor in his book
A Blood Dimmed Tide about the Ardennes Offensive noted the
climate created by the order of Sepp Diertich not to be concerned with POWs during the operation. Astor wrote that the order was meant not as a blanket authority to kill POWs but to not slow operations down because of them. Astor compared this order to one issued earlier by General Patton in Sicily. Now I'm writing from memory here, but if it serves me correctly, both orders were identical in language and tone. Astor notes that in Sicily there were subsequent shootings of unarmed Italian and German POWs as a result. This has become known (though not well known) as the Bicari Massacre. There is some basic information here on the incident:
http://en.wikipedia.org/wiki/Biscari_massacre. For more information I highly recommend
Weingartner, James (November 1989). "Massacre at Biscari: Patton and An American War Crime". The Historian (1): pp. 24-39.
Stephan