How to remove Apple Books Classics series from library?

Hello uyên274, welcome to the Apple Support Community!

To permanently remove this collection from your library, open the Apple Books app and go to the Library tab. Tap the three small dots (...) located under the cover of the Classics series or book that you no longer want to see, then select Delete. In the menu that appears, choose Hide Book (or "Hide Purchases"): this will remove the series from your view and prevent it from automatically reappearing in your library. Tell us again :)

Since the option does not appear on the entire series, open the Classics folder to see the books one by one. Tap the three dots (...) under each individual book, then choose Delete > Hide. Once all the books are hidden, the series folder will disappear on its own from your library. Always tell us again :)