If you've downloaded the book and are listening offline, please try to first remove it from offline mode. You can do this by going to the bookshelf in your app, clicking on the circle with 3 dots in it next to the title and choosing Erase offline.
Now, you can try to download the book again. Please wait until the book has downloaded 100% before you click on anything else or close the app.
If the book won't reach 100%, please make sure that you have at least 2GB free on your internal memory on the device.