I would like to study some math on my own. I am currently studying my second semester at a university and I have too much freetime so I would like to study something on my own, but I can't decide what to study. I have finished courses in: calculus in one and several variables, mechanics for engineers and a course that dealt with some number theory and proofs. I am currently taking linear algebra and probability theory.
So, I am wondering if anyone has any suggestions of what I could study on my own and if so, is there a book you would recommend?