I have installed the Reader app.
It results not yet ready for Nextcloud 13.
Looking at the Github repository Yetangitu/owncloud-apps, it seems that both the ebook reader and the https://apps.nextcloud.com/apps/files_opds are no longer maintained.
Latest commit is on 9 Apr 2017.
It is a pity, the ebook reader is such a nice app!
Is there anyone that knows the developer?
Is there any possibility to have a new maintainer of the app if necessary?