Once you've purchased an eBook, you can access and download it as many times as you want, as long as you have not deleted it from your NOOK Library.
Once your eBook is downloaded to your NOOK device or app, you will not need to download it again unless you unregister/sign out of the device or app, or if you archive the eBook/move the book to your NOOK Cloud.
This also allows you to download your eBooks on multiple devices (e.g. if you have a NOOK device and the NOOK App on your smartphone).
