Ask HN: What is your favorite resource for learning CS fundamentals?

Hey guys - I kind of "stumbled" into the IT world (I started in analytics and became a developer), so I don't really have a background in computer science, but I'm interested to learn. However, a lot of the textbooks I find online don't seem to really start from the beginning. I wanted to see if you all had any recommendations for good online textbooks or similar resources for someone in my situation?