See note on methodology Far, far in the lead here is a play by an Irish writer, first performed in 1894 and published in 1898. A lot of people who are familiar with it forget that it is set during and after the brief Serbia-Bulgaria war of 1885, the central characters being a Bulgarian officer's daughter and a Swiss mercenary fighting for the Serbs. It doesn't bear a huge resemblance to the real Bulgaria: the author's intention was to satirise British attitudes to war and class, and he pretty much succeeded. It is:
Arms and the Man, by George Bernard Shaw
After that we are into difficult territory, and I'm afraid that the next books on both GR on LT are a light-hearted Cold War thriller (not the last in that series that we shall encounter) and a short fantasy novella for young adults:
The Elusive Mrs. Pollifax, by Dorothy Gilman (second-ranked on LibraryThing, 3rd of the series)
The Balkan Escape, by Steve Berry (second-ranked on Goodreads)
Those who are familiar with Bulgarian literature will not be at all surprised (and perhaps relieved) to know that the top Bulgarian book by an actual Bulgarian writer, though some way behind all of the above, is the 1893 national classic:
Under the Yoke / Под игото, by Ivan Vazov