https://github.com/donnemartin/system-design-primer/blob/master/README.md
Found this a while ago and it has a good detailed explanaation for System Design Interviews.

1. Includes the necessary concepts.
2. Scalability of design is given good importance.
3. Relevant problems. Some of the object oriented questions could use better solutions, but it's alright.

Hope this helps. It's helping me a lot at the moment.