Getting content offline can vary widely between different sources and apps. When you're depending on an iPhone, iPad or iPod touch to get through a long flight or commute, it means downloading music, books and video directly to your iOS device. Finding books to read without a data connection requires browsing the books that are stored locally in the Books app.
Apple has consistently added features and content to its stock reader app. Originally known as iBooks, on iOS 12 and later the app has been renamed Books. iPhone, iPad and iPod touch users may notice that when reading, Books automatically enters a night theme when the room is dark. This Dark Mode can be manually controlled and customized directly in the Books app.