I learnt most of it at school, but Max Coleman's analysis of apartheid as a crime against humanity should be a good book explaining just how horrific it was. I think it's available for free here. The same website has lots of good articles too, including a basic summary of what apartheid was .

https://www.sahistory.org.za/archive/crime-against-humanity-analysing-repression-apartheid-state-edited-max-coleman

This summary, and the content in the "articles" section at the bottom of the page, is also pretty good.

https://www.sahistory.org.za/article/history-apartheid-south-africa