Not in Danish, but by a Dane and published in Denmark was Knud Paludan's remarkable book on Afghan birds: Paludan, K. 1959. On the Birds of Afghanistan. Vidensk. Medd. Dansk Natur. 122.
It summarises an enormous amount of work from the late 1940s into the 1950s and is remarkable for a host of reasons, prime amongst which is its literate English, making the task of reading the first-ever checklist of a country a joy. I take delight in picking it up and reading a page at random - the enthusiasm, passion and high academic standards are palpable.
Paludan's ornithological scholarship is evident simply because almost every one of the taxa he describes is easily related to modern taxonomic thought.
I'm eternally grateful that I was able to find an affordable, if slightly ragged copy from a book dealer in Paris; on hearing that it was wanted to help fill a gap in the OSME Region List of bird taxa, he waived postage. I think it qualifies by definition as 'a hidden gem'.:t:
MJB