Barnes & Noble gift cards can only be used with B & N (redeeming a B & N card) - so you can't use them with iTunes (ibooks, apps, music etc). According to that link you can use them to add credit to your B & N account and to buy ebooks from them, which I assume that you can then read in the Nook app (if that is available in your country's app store).
You can use iTunes gift cards to buy ibooks from Apple - they are valid in the iTunes store (music, apps, films, tv shows), ibookstore and the Mac app store.