alt Hacker NewsThe 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.