How can I provide Swift Playgrounds subscription server?

You really only need to have a web server with a few public files:

1. A json file that defines your "feed" of Swift Playground Books. Apple has documentation and a sample of what the file should look like at this url: https://developer.apple.com/documentation/swift-playgrounds/creating-a-subscription
2. Obviously, you'll need the Swift Playground Book in zip format.
3. Images to represent the thumbnail, banner, and preview images. You can find a few more details about these in the link above.

Alternatively, if you don't care to maintain a web server for this, you could upload to Github or a similar site. The book by itself would work, and even Apple seems to do this with some of its own samples, but you can also place a feed file and images in the same repo to get the same result.

I'm not sure if an exhaustive list of feeds is maintained by Apple or any other notable source, but if you want to publicize or find any other books, this is the most extensive list I've seen so far: https://github.com/uraimo/Awesome-Swift-Playgrounds. Perhaps submit it to be added there.