In 2010, I was very interested in foundations of mathematics, an extremely abstract math branches:
https://en.wikipedia.org/wiki/Foundations_of_mathematics
In particular I spent huge amount of time on:
https://en.wikipedia.org/wiki/Nicolas_Bourbaki (Set theory)
https://en.wikipedia.org/wiki/Principia_Mathematica
https://en.wikipedia.org/wiki/The_Foundations_of_Arithmetic
http://www.jhtm.nl/tudelft/tw3520/Introduction_to_Mathematic...
What attracted me is that these books doesn't require any specific knowledge of classical math. I.e. they are self-contained.
It was fun and ... the experience to delve into highly abstract view on entire math.
The big problem is that while I read that for more than a year, I had no experience in problem solving and just ignored exercises (thinking that concept is everything). As a result of that, my entire knowledge is completely evaporated and I literally can't solve any of exercises.
After that year, I dropped math till recently.
Now, I have completely different approach. I learning elementary olympiad style math and most importantly solving problems all the time. Currently, I'm into series of books:
https://www.artofproblemsolving.com/store
These books made for math olympiad preparation. While I solving exercises, I feel how solid my knowledge is.
So if you want to learn advanced mathematics, learn elementary olympiad-style math first. It will give you solid background to start learning advanced math (not just knowledge background but most importantly problem solving skills).