You may also find this paper interesting

http://wwwlgis.informatik.uni-kl.de/archiv/wwwdvs.informatik...

I find postgres source code very interesting, e.g

https://github.com/postgres/postgres/tree/master/src/backend...

Sqlite is another good one to read, and play around with.

CPython is an interesting read. The internal type system (PyObject) is pretty cool. It also uses type pruining instead of a discriminated union.