The Universe Speaks in Numbers[1] by Graham Farmelo
I found this very motivating and insightful, in terms of developing even more of an appreciation for how much math underpins other branches of science. Not that that is a novel insight by any means... but the details of the incidents where breakthroughs in mathematics allowed further advances in physics, etc. and looking at the "back and forth" between the domains, that was wildly interesting to me. Reading this book definitely helped motivate me to get serious about committing more time / focus to studying mathematics.
I also enjoyed the "counterpoint" book by Sabine Hosenfelder, Lost in Math[2]. I think these two books complement each other nicely.
Then the handful of additional (no pun intended) books that jump to mind would be:
- How Mathematicians Think by William Byers[3]
- How to Think Like a Mathematician by Kevin Houston[4]
- Discrete Mathematics with Applications[5] by Susanna Epp
- How Not To Be Wrong[6] by Jordan Ellenberg
- Introduction to Mathematical Thinking[7] by Keith Devlin
- How to Measure Anything[8] by Douglas Hubbard
[1]: https://www.amazon.com/Universe-Speaks-Numbers-Reveals-Natur...
[2]: https://www.amazon.com/Lost-Math-Beauty-Physics-Astray/dp/15...
[3]: https://www.amazon.com/How-Mathematicians-Think-Contradictio...
[4]: https://www.amazon.com/How-Think-Like-Mathematician-Undergra...
[5]: https://www.amazon.com/Susanna-S-Epp-Mathematics-Application...
[6]: https://www.amazon.com/How-Not-Be-Wrong-Mathematical/dp/0143...
[7]: https://www.amazon.com/Introduction-Mathematical-Thinking-Ke...
[8]: https://www.amazon.com/How-Measure-Anything-Intangibles-Busi...
I did the "Introduction to Mathematical Thinking" MOOC from Keith Devlin. The curriculum is available as a book as well.
The class is basically how to write and read proofs for non-math majors. It starts pretty slow, but gets harder at some point. The number theory proofs were fun.
You 'got to' grade others proofs online, and they graded yours which was an interesting way to get familiar with reading and writing proofs.
I recommend it because instead of an area of math it focuses on what it means to prove something. And the teacher is pretty entertaining.
https://www.amazon.ca/Introduction-Mathematical-Thinking-Kei...
https://www.coursera.org/learn/mathematical-thinking