Professional mathematician (but in business, not academia). I hold a PhD in mathematical physics. Below the list of books that taught me the most. However, these books are often not directly related to my current work. Also, I don't think they will be useful for everyone, as some of them are strong specialized books. These books mostly taught me to
think like a mathematician. One of the strongest skills I learned as a mathematician is to dive deep into a topic and learn almost everything that you can learn about it (going from 80% of knowledge to 99.9%). I "read" these books completely several times in my career.
* Atiyah, Macdonald - Introduction to Commutative Algebra.
* Bourbaki (in French).
* Gasper, Rahman - Basic Hypergeometric Functions.
* Hasti et al - Elements Of Statistical Learning.
* Rudin - Real & Complex Analysis.
* Thomas, Thomas - Elements of Information Theory.