This is the 12495th most frequent Russian word.
книжками
"Книжками" means "books" in the instrumental plural form in Russian.
The word 'книжками' is an instrumental plural form of 'книжка' and refers here to children's books used for comparison with regular books.
На столе лежали книги вместе с детскими книжками.
On the table lay books along with children's little books.
'Книжками' here is in the instrumental plural form, used to describe the means by which the action of reading is conducted.
Она увлеклась чтением старинных книжек из бабушкиной коллекции.
She became interested in reading old little books from her grandmother's collection.
The word 'книжками' acts in its instrumental plural case here to indicate the medium of exchange between people.