for topics that are well established, I highly recommend using a textbook! Since a college or grad level course is intended to give someone a lot of background, historical and otherwise, they tend to cover a wide range of information and can be more efficient than piecing together knowledge from blog posts and reference docs.

Systems, networking, databases, machine learning have fantastic resources available. I usually look up the syllabus of a course at Stanford or MIT and check out their recommended reading. Some professors publish their own books for free as PDFs as well.