Honestly, grabbing ebooks for offline reading is something I prioritize all the time, especially for titles from a platform like i's. My usual method involves checking the official source where I purchased or acquired the book first. Many platforms have their own apps with built-in download functions—once you open the book in their reader, there's often a little download icon or an 'Available Offline' toggle. That's the most straightforward path, and it keeps everything neat within a single library.
If I've gotten the file through a library service or a direct purchase, I sometimes transfer the EPUB or PDF to my preferred reading device manually. It requires a bit more management, but having everything in one place, like a dedicated e-ink reader or a tablet folder, makes long trips or commutes much smoother. The key is confirming the file has DRM permissions that allow for that kind of transfer, which can vary by retailer.
For ongoing series or serials, I find that apps with subscription models are particularly good about letting you stash the latest chapters. The update might require a connection, but the actual reading happens offline. It really comes down to the specific distribution method of the book you're after; the experience can differ between a completed novel and a daily-updated web serial. I usually end up with a mix, some books cached in an app and others as standalone files on my device, all ready to go the moment I'm away from Wi-Fi.