In Jolla, downloading public domain books with Aldiko is not fully functional. It downloads the book (by using the browser) in the Downloads directory but the book does not appear to the Aldiko's book self. If one tries to import that file directory to the Aldiko, the app says it cant read the file.
The fix for this is simple. Just give more permissions to the file. Adding read access for the group seems to be enough. If you have enabled the developer mode, then just go to the Downloads directory (/home/nemo/Downloads) and type:
chmod g+r the_ebook_name.epub
I tried also Nautilus for setting permission and that worked well also.