Theistic Satanism by Diane Vera - Great website with lots of interesting, well-written articles about theistic Satanism.
Demonolatry website - Resource site for Demonolators.

Online libraries and archives

Demonolatry library and archives
Twilit Grotto: Archives of Western Esoterica
Internet Sacred Texts Archive
"OTO" section on The Textfiles Directory - Contains lots of Aleister Crowley's writings in text file format, but also other files related to Satanism more generally and the "Satanic Panic" scare of the '80s and '90s.
Internet Archive
Project Gutenberg

Search engines

DuckDuckGo - The search engine that doesn't track you.
Marginalia Search - "The search engine calculates a score that aggressively favors text-heavy websites, and punishes those that have too many modern web design features." Ideal for finding interesting personal websites or older sites.
NeoSearch - A search engine that only searches Neocities sites.