To download an entire eBook to any device, you must also download the free Adobe Digital Editions (ADE) software/app and have (or create) an Adobe ID. (An Adobe ID is different from your Assumption ID.) When you download an eBook, it opens in Adobe Digital Editions, and you can read your book offline. After the checkout period is over, it remains in your ADE library with a status of Expired. To continue reading, you have to download it again.
Here is a helpful video about downloading Adobe Digital Editions on a computer. (Note, the process is similar on a mobile device--you still need to download the ADE app from your device's app store and create an Adobe ID.)
Please see our Troubleshooting Tips for more information on downloading to MacOS, iOS, Windows, or Android