Perform the following troubleshooting steps:

  1. Check that you have the latest version of Nokia Reading app on your Lumia device.
  2. Check that the date and time are set correctly on your Lumia device.
  3. Check that your devices network connection is working with other applications (for example with Browser or Email).
  4. Check that the network (mobile data) signal coverage is good. For example you shouldn't download books when travelling by train.
  5. Reboot your device and try to download again.
  6. Try to download via Wi-Fi instead.
  7. Try to download again later.

If the above steps do not help then contact Nokia Care.