There are many important components (CAP, Load balancers, DNS, Availability, Consistent Hashing, TCP/IP etc.) in any system design interview, some also dive deeper into the database storage engine, need to explain the algorithms and data structures that the storage engine uses on disk, and how it affects the whole system performance-wise.
I am trying to compile a list of these resources . The typical suspects are always these two
http://highscalability.com/
https://github.com/donnemartin/system-design-primer
I have read also Designing Data-Intensive Applications book.
Please feel free to recommend some of the things that you found rewarding in your journey as an experienced developer.