I was just skimming through the NCL App as I'll be on the Prima in 3 weeks and found that they have a digital library. If you download the Libby app you will have access to tons of books and magazines. On Embarkation day login with your lattitudes number and you'll be able to download one book and place a hold on an additional book. You can use your phone, tablet or e-reader. You have access for 7 days or the duration of your cruise.
I have the Libby app and I use it in conjunction with my library card, I get access to free audiobooks as well but have access for 3 weeks. It works with the kindle app.