When you tell the container that /mnt is pointing to /mnt/user/Books in unraid then the container will see the contents of /mnt/user/Books under /mnt. In other words, in ubooquity you should set the share location of your books to just /mnt because you've already told the container that /mnt is pointing to /mnt/user/Books and ubooquity will see all of your content under /mnt. Does that make sense?
If you wanted you could set /mnt to /mnt when creating the docker and then in ubooquity you would have to specify the full path. But the ubooquity container would also be able to see everything which I guess could be less secure. Especially if you expose ubooquity to the internets.
Worked perfectly! Many thanks!