This is the 7942nd most frequent Russian word.
книги
Books.
In this sentence, 'книги' is used as the plural form of the noun 'книга', meaning 'books', referring to reading materials.
Я читаю книги по истории.
I am reading books on history.
Here, 'книги' is also the plural form of 'книга', used to describe multiple books located on a shelf.
На полке лежат книги.
There are books on the shelf.
In this instance, 'книги' is in the genitive plural form ('книг'), reflecting possession and uncounted quantity.