If all the copies of a digital book are checked out, you can place a hold on it instead. This will put you on a wait list for the title.
Once the title becomes available, you'll get a notification.
On iOS and Android devices, you'll get push notifications about your loans and holds by default.
You can manage each type of notification individually. Here's how:
You can find all of your holds by going to Shelf > Holds. From there, you can see where you are on the wait list for each hold, and edit or cancel holds as needed.
After you place a hold, go to Shelf > Holds and tap to see your place in line. You'll also see your approximate wait time, the number of copies in use, and the number of people waiting.