From the longer Wikipedia page 
The SS. Cyril and Methodius National Library (Национална библиотека „Свети Свети Кирил и Методий“) is the national library of Bulgaria, situated in the capital city of Sofia. Founded on 4 April 1878, the library received the status of Bulgarian National Library three years later and the Bulgarian National Revival Archive was merged into it in 1924.
It is named after "SS. Cyril and Methodius". They are the founders of the Cyrillic alphabet.
The present building of the library is among the landmarks of Sofia. It was designed by the famous Bulgarian architectural team Vasilyov-Tsolov and completed in the period 1940-1953.
More information on the Bulgarian Wikipedia page 
The website, in Bulgarian, is