After this coming semester, all or almost all of the math courses that you take will be heavily proof-based. Analysis, topology, and algebra all involve a lot of reading and writing proofs. So I would recommend using this time to become acquainted with mathematical proof techniques. Hammack's Book of Proof is freely available online. I haven't read it myself but I've heard good things about it. Eccles's An Introduction to Mathematical Reasoning is another good choice and it is the intro-to-proof book that I learned from.
Whatever books or resources you choose, just remember that it is very important to do the exercises.