There are plenty of references available over web on System Design mandates or valid references. But I would like to list down few in here which are of my choice. It gives very clear view and directions on good system design.
In Generic
https://github.com/checkcheckzz/system-design-interview
Few Key Points To Note
https://hackernoon.com/fundamentals-of-system-design-part-0-b3e2fe45daec https://hackernoon.com/fundamentals-of-system-design-part-1-c87b1d2bfd31 https://hackernoon.com/fundamentals-of-system-design-part-2-abbe437ce2dd https://hackernoon.com/fundamentals-of-system-design-part-3-8da61773a631 https://hackernoon.com/fundamentals-of-system-design-part-4-d6a62f3fa779
Cheat Sheet
https://gist.github.com/sughosneo/f93c18ba1c9c41c0b7f4bdd3d274acb6
Popular Tools
Different diagrams always help to understand an system architectural design better. Here are few of the important tools what users like to use. https://www.quora.com/What-tools-are-used-in-software-design https://www.tutorialspoint.com/software_engineering/software_analysis_design_tools.htm