High school student here...
Recently I was told I could go to the book store and pick out any math books I want. (2 or 3)
Does anyone have some good suggestions? I'm comfortable with anything involving calculus and I am currently studying linear algebra. It doesn't really even need to be a textbook just something that will help me learn and build mathematical maturity.
Have you seen What is Mathematics? by Courant and Robbins? It is by far the best introduction to mathematics that you can get your hands on, in my opinion. Here's a short form of the table of contents for your reference: