Hello, I suggest you to buy "the history of bulgarian air power" of Dimitar Nedialkov published by Albatros MDV in 2013.
I bought this very good book of 479 pages full of pictures and facts about the history of this air force from the beginning to our times last year in France at LELA presse (see on the web
www.avions-bateaux.com) for 39 euros
regards
Alain