curl -s https://standardebooks.org/opds/all | grep -oE "/(.*).azw3" | sed -e"s/^/https:\/\/standardebooks.org/" | xargs -n 1 curl -O
P.S. Calibre seems to only work with them and the kindle when you send over USB (I keep my paperwhite in airplane mode, signed out, ads disabled)[0]
edit: If you use MacOS, Yamamoto Mitsuharu‘s emacs-mac [3] is my recommendation, as for linux the version from your distros package manager should be apt. ;)
[0] https://github.com/hlissner/doom-emacs
[1] https://github.com/syl20bnr/spacemacs
[2] https://github.com/emacs-evil/evil
[3] https://bitbucket.org/mituharu/emacs-mac