[SteveMoody73]

wordI think it can be hard to recommend a particular codebase, well written code can be good to read but if you want to become better at a language or problem domain then sometimes reading badly written code may be a better way to learn.

Working through some badly written code that actually performs well can be a real eye opener. I mainly work in C and reading some legacy code (sometimes even my own) can be a challenge to work out exactly what's going on.

If you want to learn how an algorithm works, then a good clean codebase with lots of comments is a good way to go. If you want to learn the details of a particular language, then just read a lot of code in that language whether it’s good or bad.