Send me a CC debug log as described
in this FAQ.
Sometimes the problem is caused by the reusing an SD card from a previous device, which can cause the CC "app private" folder (Android/data/com.multipie.calibreandroid) to have incorrect permissions. It might be sufficient to uninstall CC, delete that folder (somehow), then reinstall CC. This would make it impossible for CC to find the one-true-path to the books folder, which would cause CC to give up.