Russian and Bulgarian Originals:
• А.И. Изотов “Старославянский и церковнославянский: Грамматика, упражнения, тексты”
• “Древнерусский язык” (университетская методичка без автора)
• Стоян Стоянов, Мирослав Янакиев “Старобългарски език”
(In English: A. Izotov “Old Slavonic and Church Slavonic. Grammar, Exercises, Texts”
“Ancient Russian” (university syllabus without authorship)
“Old Bulgarian” by S. Stojanov, M. Janakiev)
Old Bulgarian and Old Slavonic are actually the same thing: it’s the first attempt to codify the ancient Slavic language in written form. Church Slavonic is its subsequent development.
Both languages were bookish and never used practically in everyday life. Church Slavonic is still used by the Orthodox church.
AncientRussian is the codification of the really spoken language (11th-14th centuries).
Three books to give you some notion of what the languages looked like in 8th-14th centuries.
First published in the Levinarium Telegram Channel (now closed and deleted)