Today I started reading Introductory Graph Theory by Gary Chartrand. It's starting off teaching about modeling. Modeling in this case is turning something into math, such as the area of a square being A=W^2.
I reached the part in the book where it talks about actual graphs, but it was using terms from set theory which I'm not familiar with. Fortunately the book has a section in the back which teaches you the basics, so I started going through that as well. It's mostly been review so far (I picked up some bits of set theory here and there while studying other things), introducing you to the symbols/operations such as "element of set", "subset", "equal set", etc.
This made me curious about whether there is a good book devoted to set theory. After some searching I found this thread which contains an abundance of recommendations. Naive Set Theory by P. R. Halmos and Theory of Sets by E. Kamke look interesting, the first covering more area in less time and the second covering a few topics in more depth.