Books: Operating Systems/Database/Networking/Computer Security/Computer Architecture textbooks, Software Engineering textbooks (Clean Code, Design Patterns, Designing Data Intensive Applications, Domain Driven Design as a short list off the top of my head)
"Designing Data Intensive Applications" is an absolute goldmine for things like message queues but also going beyond understanding the full implications in database selection and other common, distributed-oriented engineering decisions modern software engineers may come across.
By the same token "documentation". Today I explained to a colleague how they could enable port-forwarding across their already-open-SSH-session via "~C".
Every now and again I pick a tool I use a lot, and read the man-page. Things like "less", "bash", "ssh". Complex enough to contain surprises, but simple enough that you take them for granted.
