Since Devuan is mostly just Debian without SystemD, most things don't need separate documentation, and a lot of what you read in sources like the Debian Administrator's Handbook https://debian-handbook.info/browse/stable/ should still apply.

If anyone has actually created a centralized guidebook to "Administering Systems Without SystemD", I'm too green to know about it myself, and would like to see it. Devuan supports multiple init systems, so some things aren't going to be the same even across all Devuan systems.