there are several German language KG histories from Motorbuch Verlag still available;
KG-4, KG-51, KG-55 and KG-100 have been done by various authors and there are newer histories in the VDM range of books available as well.
you might also look for a copy of "Das Kampfflugzeug He-177 und seine Weiterentwicklung" which also contains a good overview of the Greif and its use during attacks on the UK.
should you be interested but unable to locate a copy, let me know
cheers
phil
oh before I forget, this here might also be interesting for you, once it gets published:
http://www.ianallanpublishing.com/pr...at=1003&page=5
cheers
phil