This is the 12494th most frequent Russian word.
книге
"Книге" is the dative or prepositional case of "книга," meaning "book."
The word 'книге' is in the prepositional case, indicating location (inside the book).
Она читала рассказ в книге.
She was reading a story in the book.
The word 'книге' is in the prepositional case, indicating the topic of discussion (about the book).
Она подарила мне заметки о книге.
She gave me notes about the book.
The word 'книге' is in the prepositional case, denoting the source of information (the book).