Available online as a multi-page HTML or an e-book in EPUB format.
You can also build them locally:
nix-build release.nix -A html-split && firefox result/share/doc/nix-pills/index.html
Similarly, for an EPUB version, run:
nix-build release.nix -A epub && foliate result/share/doc/nix-pills/nix-pills.epub