Ask HN: What books or courses can help build a strong base in Computer Science?

I am reading HtDP (https://htdp.org/) at the moment. I am quite liking its style. What to do after that? I want to build a solid base of CS fundamentals. Like programming high level as well as low level, different paradigms, algorithms, basics of TCS and other applications.

What are some good foundational books in each area that teach timeless concepts and not just some fancy tool that is currently popular?

If there are available online courses (not like those shallow ones in MOOC platforms) with public materials let me know about them too.