One of the most challenging exercises in any language is to learn how to organize your code and modularize it at a large scale. I recommend reading Postgres's s

One of the most challenging exercises in any language is to learn how to organize your code and modularize it at a large scale. I recommend reading Postgres's source code. But don't wander aimlessly, set a goal. For example, figure out how an sql select query is parsed and processed, how types are deduced, etc and learn the programming patterns applied as you go through. I often recommend postgres because it's highly commented, and easy to understand.