For Hooks (IMO the most powerful feature) Id recommend only https://code.claude.com/docs/en/hooks-guide and for Plugins, Skills, MCP and so on the official documentation by Anthropic has been the source you'd need. As for harnesses and CLI tools Id go with whatever you're already familiar with, can't make a particular recommendation.
Harnesses, which kick off agents with what to do.
Tools, which show an agent where in a process it is, and what the next step should be.
In my experience I find Hooks to be extremely powerful cross-project. CLI Tools are easy to make also, and work really well for guiding agents.