Let me start that documentation varies based on the type of the project, the kind of documentation, targeted audience. For example, we have two kinds of documentation - internal, which is targeted for developers and is highly technical and external (or public), which is divided into two parts - one is business oriented and the second is a developer oriented. All these documentations have different narrative styles, serve a different purpose and are aimed at different target groups. So, you'll have to keep that in mind.
Now, that this is out of the way, let me share a couple of resources I've gathered through the years:
This is the go-to place for technical writers. It's a community of people who care about documentation. They have Slack channel, learning resources, like a newsletter, podcasts, videos, documentation guide (which you should definitely check), blog posts and much more.
I'm not a big fan of books about documentation. I have tried reading a couple of books about technical writing, which left me disappointed - they were all too narrow and didn't give any advice. Probably I didn't choose the right books back then. That being said, I've heard good reviews about Technical Writing Process and Elements of Technical Writing, so you should definitely check them out.