I've read the basics of this branch and I found it extremely interesing, and I would really love to learn more about it.
I want to study as much as I can on my own, as my course doesn't have group theory, unfortunately.
What would be a good (ground-up level) introductory book, and then a mid-level group theory book?
Thanks so much in advance.
E: I'm taking a number theory course at the moment. I haven't taken any other math courses yet, although I've some studied calculus, real analysis and linear algebra on my own.