
"This text leads the reader through the various tactics involved in solving mathematical problems at the Mathematical Olympiad level. Solving Mathematical Problems includes numerous exercises and model solutions throughout. Assuming only basic high-school mathematics, the text is ideal for general readers and students 14 years and above with an interest in pure mathematics."--P. [4] of cover.
In the spirit of OP's question:
How to Solve it by G. Polya
Solving Mathematical Problems by Terrence Tao
Introduction to Mathematical Thinking by Keith Devlin
Are all amazing, How to Solve it in particular is an all time classic.
- Mathematics and Its History (John Stillwell)
- Journey through Genius: The Great Theorems of Mathematics (William Dunham)
- Proofs Without Words: Exercises in Visual Thinking (Roger B. Nelsen)
- The Mathematical Universe: An Alphabetical Journey Through the Great Proofs, Problems, and Personalities (William Dunham)
- Odds & Ends: Introducing Probability & Decision with a Visual Emphasis (Jonathan Weisberg)
- Introduction to Probability (Joseph K. Blitzstein, Jessica Hwang)
- The Secret Life of Equations: The 50 Greatest Equations and How They Work (Richard Cochrane)
- Euler's Gem: The Polyhedron Formula and the Birth of Topology (David S. Richeson)
- 3Blue1Brown Essence of Linear Algebra: https://www.youtube.com/playlist?list=PLZHQObOWTQDPD3MizzM2xVFitgF8hE_ab
- 3Blue1Brown Differential Equations: https://www.youtube.com/playlist?list=PLZHQObOWTQDNPOjrT6KVlfJuKtYTftqH6
- Dynamics: The Geometry of Behavior (Ralph Abraham, Christopher Shaw)
- Geometry, Relativity and the Fourth Dimension (Rudolf Rucker)
- 3Blue1Brown Essence of Calculus: https://www.youtube.com/playlist?list=PLZHQObOWTQDMsr9K-rj53DwVRMYO3t5Yr
- Calculus for the Practical Man (J.E. Thompson)
- Div, Grad, Curl, and All That: An Informal Text on Vector Calculus (Harry M. Schey)
- Who Is Fourier? a Mathematical Adventure
- Physics, Topology, Logic and Computation: A Rosetta Stone (John C. Baez, Mike Stay)
- An Illustrated Guide to Relativity (Tatsu Takeuchi)
- The Mathematics Of Quantum Mechanics (Martin Laforest)
- A Student's Guide to Maxwell's Equations (Daniel Fleisch) or if you're also interested in a visual guide, I wrote one as well: https://github.com/photonlines/Intuitive-Guide-to-Maxwells-Equations
- Introduction to Elementary Particles (David J. Griffiths)
- An Invitation to Applied Category Theory: Seven Sketches in Compositionality (Brendan Fong, David I Spivak)
- Solving Mathematical Problems: A Personal Perspective (Terence Tao)One thing the article doesn't touch on is Tao's prominence in massively collaborative math research through his blog (latest such work from there was discussed on HN a day ago https://news.ycombinator.com/item?id=21542054). This new approach, first proposed by Tom Gowers (https://gowers.wordpress.com/2009/01/27/is-massively-collabo...) in 2009 has successfully solved a number of open problems in the past ten years (https://polymathprojects.org/2019/02/03/ten-years-of-polymat...).
But to me the even more amazing thing is how good an educator he is. Most mathematicians I have encountered were of the "proof is left as an exercise" type, where if you didn't see how things done would't or couldn't help you in learning how to proceed. Tao is a big outlier in this regard. His How to Solve Mathematical Problems (http://www.amazon.com/Solving-Mathematical-Problems-Personal...) is more focused than Polya's famous book but is a much better help in learning how mathematicians think. You can get the first chapter for free (http://www.math.ucla.edu/~tao/preprints/problem.ps), an excellent way to finish off a Friday.
Now, if money's no object, i.e. you're willing to spend around $200, the gomboc (http://en.wikipedia.org/wiki/Gömböc) is a wonderful object to buy, you can buy it at the gomboc-shop.com