Currently gnome-books are available in the system, however, it is just a preview of the features that this application hopes to have, since Evince is currently a better alternative for reading PDF, comics and other document formats.
According to the gnome-books roadmap, support for the future epub format is expected, but nothing more. It would be a good idea to consider another ebook reader that is functional and supports a greater number of formats.
I think a good option right now is Easy eBook Reader. Additionally, the author commented that in the future he could create a Flatpak package of his application, so cooperation between projects could exist. I think it is a better method to encourage the use of a new technology.