This is the 12496th most frequent Russian word.
книжная
Relating to books.
Here, 'книжная' is an adjective modifying 'полка' (shelf), meaning 'book' or 'for books', indicating the shelf is used for holding books.
Её книжная полка заполнилась новыми романами.
Her book shelf has been filled with new novels.
In this sentence, 'книжное' modifies 'издание' (edition) as an adjective, suggesting that the edition is in book form.
У неё появилось книжное издание любимой сказки.
She got a book edition of her favorite fairy tale.
The word 'книжные' in this context is an adjective modifying 'иллюстрации' (illustrations), referring to illustrations that are intended for books.