It's in Cyrillic. The official title is "Четиризначни математически таблици и формули".

googled Четиризначни because didn't recognize it as a russian word (it means four digit) and the first hit is that book

https://www.book.store.bg/p35401/chetiriznachni-matematiches...

Bulgarian is a distinct language using the Cyrillic alphabet (as is for instance Macedonian, Serbian and Ukrainian)

wikipedia used to have this

https://i.redd.it/5ibk260ssqa61.png

e.g. i understood the other two words in the title no problem.

>Ukrainian

i'm from moldova and i speak russian and understand ukrainian pretty easily. calling them distinct languages is true but it's pedantic - it's a very blurry line how mutually intelligible the two languages are.

-Similar story where I come from - Norwegian, Swedish and Danish are sufficiently similar that we can mostly understand each other without much effort, especially (my opinion) in writing - Danish pronunciation is far more challenging to grasp than Swedish, at least to my Norwegian ears.

It's not Russian language. Close, but it's not.