The Kobo version is a DRM free epub, so you can read it wherever you like. I'm still figuring out how/where to sell a PDF, because I really, really, _really_ do not like epub for technical books. It's ok for novels, at best. Tips welcome.
https://berksoft.ca/gol/genius-of-lisp-chapter-8.pdf has a free chapter.
Polar.sh's pretty easy to set up! It was originally built with selling download links in mind (for stuff like OSS software).
Not affiliated, just a happy customer.
Turns out that for me, Shopify was even easier, so that's sorted now. In any case, thanks for pushing me into the right direction.
I'm gonna check it out, thanks.