Beyond assigned college textbooks, the book that influenced me the most and prepared me for higher mathematics was "An Introduction to Mathematical Reasoning" by Ecceles:
http://www.amazon.com/Introduction-Mathematical-Reasoning-Peter-Eccles/dp/0521597188
It starts off simple that any high school student can understand and go through in topics that are somewhat familiar or at least easy to grasp. The beauty comes from doing the problems at the end of each section. A lot of them are easy to understand and accept, proving them is a bit challenging.
I learned the basics from Peter Eccles's An Introduction to Mathematical Reasoning. The primary focus of the book is on proof techniques, not set theory, but I think it presents the very basics of sets and functions in a nice cohesive way. I recommend it as both a very basic introduction to sets and as good preparation for upper division proof based math classes you will probably start taking soon.
If you are looking for a more advanced reference I would recommend going to your university library, finding the section where the algebra and set theory books are and just check a bunch of them out. Try reading each of them and return the ones you don't like. Rinse and repeat until you find one that clicks with you.
The class I took "Math Reasoning", which I believe is equivalent to the one you're taking used this book. www.amazon.com/Introduction-Mathematical-Reasoning-Peter-Eccles/dp/0521597188/.
I cannot, however, give you any advice on how it compares to your book, as I'm not familiar with yours.
https://www.amazon.com/Introduction-Mathematical-Reasoning-Numbers-Functions/dp/0521597188
Would easily be the best value for $35.33 you'd ever get. Pound-per-pound in terms of word economy, while giving mercy to let you get creative mathematically.
Can't remember much from my course, but going back through it, the first few chapters were fun. Doesn't make you fuck around with set theory notation beyond what is necessary and gets to the heart of what math is. Or at least shaped my understanding of it.
Will you know how to prove Green's theorem by the end of this book? No. Was the process of reading it an enlightening one for me, yes. To be fair, my course skipped some of the denser material, we definitely didn't cover prime numbers. Like I said, I don't remember everything that was covered, so if your hobby includes collecting wikipedia entries, count yourself out of this one or let it show you the futility in that endeavor.
That said, if a function based understanding of calculus is what you desire, there is merit in that path as well. I found differential equations to be very rewarding, but it's been years since I learned it so I can't even tell you what a differential equation is.
Sadly I have no use for my copy anymore. If anyone wants it, I'll even buy the flat-rate shipping box. But then I know your address. I'll put mine on there too. :)
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Seriously, if you can buy it on amazon instead, lol.
There is actually a book called An Introduction to Mathematical Reasoning.