[jordibc]

For sure I got significantly(?) better with classics like Spivak, Apostol, Rudin.

"Real and Complex Analysis" by Rudin, and the two books both named "Calculus" from Spivak and Apostol. But also from Apostol his more concise and far-reaching "Mathematical Analysis". And from Spivak his small gem "Calculus On Manifolds" made quite a dent on me.

Other than more "classic math" books, I also wanted to mention two outliers that I found eye-opening and generally awesome:

* Street-Fighting Mathematics, by Mahajan (http://streetfightingmath.com/). Intuitive, useful and fun.

* Geometric Algebra for Physicists, by Doran and Lasenby. I found the power and elegance of geometric algebra mesmerizing, and even if this book is also about physics and there may be more appropriate math-only books about geometric algebra, this is the one that made it for me.

[dorchadas]

> * Geometric Algebra for Physicists, by Doran and Lasenby. I found the power and elegance of geometric algebra mesmerizing, and even if this book is also about physics and there may be more appropriate math-only books about geometric algebra, this is the one that made it for me.

I've tried to read several of them, and, sadly, I feel most geometric algebra books fail at explaining it. It's a shame as it's part of what kindled my interest in pure mathematics and I still feel I'm nowhere nearer understanding it despite working through several other mathematics textbooks, including just plain algebra. But, it did spark my interest and now I've moved on to other interesting topics, though Geometric Algebra is still my white whale.

[jordibc]

In case it helps:

In addition to "Geometric Algebra for Physicists" (whose first two chapters I'd recommend to get a nice overview), I found Hestenes' "New Foundations for Classical Mechanics" to be very good and readable.

Also, there are many good resources in https://bivector.net/ , including videos, papers, presentations and programs.

Finally, an interesting paper (that got me kickstarted) is "Imaginary Numbers Are Not Real—The Geometric Algebra of Spacetime" by Gull, Lasenby and Doran.