http://matt.might.net/articles/what-cs-majors-should-know/

To add a few of my own that aren't mentioned here which are more in the vein of what you're really looking for (disclaimer: I have not read most of these all the way through, some at all.)

Compilers: The Dragon Book, the dragon book is usually criticized for being outdated so try and find another compilers book if you want, what you're looking for here is switching your mindset from 'I will try to hack up a compiler.' like the other comments here seem to be recommending, which will never work, to going 'this is a complicated piece of software it wouldn't be intuitive to me how to build, is there a theory behind it?'. Quite often the answer is yes.

Operating Systems: The dinosaur book.

Databases: At the University Of Washington I was told to read Database Systems by Garcia-Molina: https://www.amazon.com/Database-Systems-Complete-Book-2nd/dp...

To talk a little bit more about what recommends this approach over other ones:

The other comments seem to be recommending a mindset that your first step to trying to understand these complex systems is to implement them from scratch with your own naive view of the problem. This is at best a good beginners exercise to teach you the value of humility and learning from the work of others, or perhaps to get some practical experience in why the problem is hard. In general, you are never going to be able to have one of these systems spring forth from your brow on its own strength, unless you are a genius beyond genius who can in days or weeks replicate the hard work of years of study by some of the smartest people in your field.