The professor of the history class I am currently taking, in which I am doing research on Reinhard Gehlen, has assigned a historiography paper with a minimum of 20 secondary sources. This would not be a problem if that many secondary sources existed on Reinhard Gehlen. I have been able to only find about 12 books and articles about him and his intelligence-gathering organization. Everything else I have found has been primary.
I would say the biggest reason for this is because the CIA has only recently (within the past 5-6 years) declassified and released material about Reinhard Gehlen and the Gehlen Organization during the Cold War. That isn’t sufficient time for 20 books to be written on the subject, which means I have stumbled into relatively untreaded ground. Most of the books and articles I have found were actually written in the early 1970s shortly after Gehlen retired as president of the Bundesnachrichtendienst (the Federal Intelligence Agency which his organization essentially turned into after its adoption by the West German government).
I certainly can’t complain about the abundance of primary sources that I have found and seem to still be unwritten about, but it still does not help my situation with my historiography paper.