I consider pic related to be one of the best novels from a Bulgarian writer. The commie regime made sure to translate it in many languages because it was used for propaganda (the novel deals with the forced islamization of a Christian region in the 17th century and the commies in the 1980s were trying to do the reverse to the Muslims in Bulgaria). But apart from the novel being used for shitty politics it's actually very good, it has an interesting style, plot and characters, I love it.
There are a couple of other novels I would consider top-level but yeah, nobody bothered to translate or popularize them abroad. I think all small countries have some gems that are never going to get recognition outside their borders. There was this quote from a book with interviews with Umberto Eco and Jean-Claude Carriere and Eco said something like "There could never be a great Bulgarian poet", meaning that works in small languages are doomed to never have the reach of the big languages like English, French, Italian, etc.