Question: I've recently finished a MS in computer engineering and long term it's likely that I'll look into doing a PhD. One of the reasons I didnt do it straight away is that I dont really have a clue what I'd like to do it in. I enjoyed parts of EE, CE, applied mathematics, and CS. 

Both to maybe look for what areas I'd most like and just for general knowledge, I'd like to buy a few textbooks and self study topics. However, I've got no clue what's... next?

Where can I find what's next, if that makes sense? Like, in undergrad, you take an intro circuits class, then Electronics 1 which introduces transistors, then Electronics 2 which has advanced amplifier circuits etc. It builds. I'm not sure what builds on what I've studied from here. I understand these topics "build" in undergrad in a cleaner, linear way since they're designed to, and now theres a plethora of options, but what are they? Where do I start looking?

By the end of my degree, I was just reading course descriptions and taking whatever sounded interesting/applicable to my thesis. This worked extremely well. Now that I'm graduated, however, I dont have easy access to up to date course listings. I've been reading random course listings at universities, but they're not helpful, and dont always list textbooks. 

For reference, my favorite courses at the end of my degree were:

Digital signal processing, Dynamic optimization (optimal control), Linear systems theory, Intelligent systems, Compiler theory, and Nonlinear optimization 

Let me know if this would fit better elsewhere.

Answer: 
You can search university reading lists, e.g., [https://www.google.com/search?q="reading list"%20"applied mathematics"%20site:edu](https://www.google.com/search?q=%22reading%20list%22%20%22applied%20mathematics%22%20site:edu), replacing *applied mathematics* with phrases to hone in your search.
See the schools you are interested in, check out the descriptions of their MS/PhD programs. There you should see what graduate classes are offered. Their reading lists should give a starting point.