If you set it up as just local sharing, ISP can't see. If it's public IP, in theory, since you don't seem to use HTTPS, they can. However, they won't.
I think people ise Calibre with Send to Kindle email feature, not the content server. I find that somewhat pointless as I have to wait for it to show up on my kindle when I can just use USB to make it show up instantly, as a format that I choose, without using any Amazon's server. In fact, I have never turned on the Internet access for my kindle.