I'm a graduating senior who found out much too late in my academic career that I absolutely love mathematics. I've taken all the basic calculus courses, a three quarter series on real analysis, abstract mathematics (essentially a proof course), and an applied linear algebra course. However, the university will not let me stay long enough to continue and get a major or even a minor in mathematics. Because of this, I would need to extend my study beyond coursework. This leads me to ask : What texts anyone would recommend to read for someone like me, with a basic understanding of theory and proofs, in order to better understand mathematics? I'm much more interested in theory than application, especially since my major is economics and there isn't much application of mathematical theory in my field beyond the courses I've already taken. I was wondering if perhaps anyone knew of some interesting or foundational books I could use to further my study of mathematics. or if there happens to be anyone who knows of a text on the application of mathematical theory to economics beyond the introductory calculus series? Thank you for your responses.