Quite a lot. I read many times "Designing data-intensive applications" which I highly recommend. There's a book called "system design interview" I believe, that is a summary of the most typical designs. There are also a bunch of videos on youtube. I read some research papers of classical designs. I played with some typical components, such as nosql dbs. I even implemented some prototypes.
Not on the top of my head, but they are usually cited in the blogs or video that present some classic systems. And "designing data intensive applications" has all the references. That being said, I don't think it's worth getting that deep for system design interview preparation unless you're already quite advanced. Retrospectively, I think I spent too much time on advanced material, overestimating what was required for this roles.
