Ask HN: Recommendations for high quality, free CS books online

I recently stumbled on https://en.algorithmica.org/hpc/ which I absolutely loved. It's really well written, comprehensible and concise. It felt like a pleasure to read which I find really rare with CS textbooks and I feel like I've come out of it understanding how computers work a bit better

Does anyone have any similar CS books they'd recommend? Ideally they'd be: - preferably online and free - aren't insanely dense in terms of length (500+ pages) - are written with some sort of consideration towards being comprehensible to the user rather than just dumping huge swaths of math and terse text on them

I'd say that last point is particularly important and something that I think the Algorithmica work did really well.

Thanks!