You can use published analysis but you should shy away from things that specifically use numerical engine analysis. Using published analysis that has been checked by computer would be fine. It would also be a good idea not to use a source that heavily relies on lists of variations given by the computer, since that is essentially using an engine to give moves.
For correspondence chess matches, the rules state that it's acceptable to look up positions in a database of games, and by extension to look over similar expert games to see what the pros would do in similar positions. Analysis, however is banned.
I like to research my openings and positions pretty extensively when I play daily chess games. I'll read up on openings that I'm not familiar with to figure out their general principles, and comb through the database in search of promising lines. This feels in line with what's acceptable in a correspondence game, but I'm not sure about where one type of research falls in terms of ethics, namely looking up published analysis of professional games.
What's the general consensus on reading published analysis, either by an expert or with computer assistance, on grandmaster chess games that are similar to games you're playing? Is this acceptable research, or does it fall under "computer assistance/outside help"? If I find an article by Silman or a video by Simon Williams talking about and giving analysis for a specific game that's either identical or very similar to one that I'm playing, can I read or watch that as part of my study, or is that cheating?