This is the 1459th most frequent Russian word.
книг
The word "книг" in Russian is the genitive plural form of "книга," meaning "books."
'Книг' is used here in the genitive plural case to denote possession or quantity.
У меня есть много книг.
I have many books.
'Книг' is used in the genitive plural case due to the prepositional phrase 'одну из'.
Я читаю одну из этих книг.
I am reading one of these books.
'Книг' is in the genitive plural case because of the construction 'полна чего' (full of something).