I am obliged to point out that there is a mini-history of this unit in our book "Bomber Units of the Luftwaffe, A Reference Source" by H.L. deZeng and D.Stankey, published by Midland Publishing (Ian Allan). This is not a full blown history, but it is an overview of formation, equipment, operations and commanders. The KG 53 history covers 15.25 pages including photos. For a detailed review see:
http://sonic.net/~bstone/archives/080330.shtml.
Apart from the Kiehl book, there are no other published histories, only incidental references to it scattered over about 4 dozen books.
It is available just about everywhere. Try the public library first.
I hope this helps.
DGS