Thanks to Paul Anderson, I picked up recently on ebay this book from Bulgaria. It may be missing its cover but it is still a great find. Disney items from Bulgaria from before 1950 are extremely rare and this book from 1946 seems to be an oddity, as explained by my friend from Serbia Sasa Rakezic:

[This is from Bulgaria indeed. Great find. It says: MIKI MAUS PLANINAR (Mickey Mouse Mountaineer), ILUSTRACIOT OT UOLT DISNEI (Illustrations by Walt Disney), IZDAETELSTVO "ZNANIE" SOFIA (Published by Znanie (Knowledge), Sofia).

Moreover, on the inner cover it's marked that it's number 24 of the "Miki Mouse stories" of the "Cheap Book " edition. It's not clear if that is part 24 of an edition only dedicated to Mickey Mouse stories, or (which is more likely) it is just one of the books dedicated to Mickey Mouse in an edition of different stories. This is all that I was able to guess... (The name of the translator is Vyara Mak, if that is of any significance to you).

The book was approved by the Ministry of People's Education. As an addition, they added a story by Leon Tolstoy! I can't tell which story it is, because that part is missing, but it's interesting that the new ("Communist") government mixed stories by Disney with Tolstoy!]