Microeconomics, back in college. It's the stuff you've always thought of, but have never been able to express in a succinct way. At least for me :)
Never Split the Difference: Because you have to negotiate in life. Nothing feels worse than realising you made a bad deal with an ugly institution/individual. The book shows how to use your leverage in negotiations, and you can easily scim through it in a weekend.
Operating Systems: Three Easy Pieces + Linux Device Drivers 3rd: All the cool tech in my opinion is made with systems-programming. Game-engines, Risk/Pricing Engines, Radar/GPS, Compilers, VM's, OS's, probably also Apples AR goggles are made with systems languages, so likely/sadly C and C++. These two books are real and practical. The Linux one is outdated, but it worked for me.
Oppenheimers books on Signals and Systems: Because signal-processing. And the Fourier-Transformation is the most unintuitive AND useful piece of mathematics I have used/seen. (Stochastic Integrals is a close second)
Differential Equations. ODE's, SDE's, PDE's. I don't really have a good book here sadly, but Differential Equations describe the world.
This is very basic but I read both at a young age and I'm massively happy I did.
1st one being "Thinking fast and slow" by Daniel Kahneman - completely reshaped my model of how much we are in control of our thought process.
2nd one was "Thus spoke Zarathustra" by Nietzsche - which is a very difficult read. Overall Nietzsche's approach to philosophy just immediately resonated with me and sent me down a path of thought that's still the foundation of my world model today. The whole approach of seeing life as inherently meaningless so meaning can be chosen at will was a great thing to have growing up. You definitely don't need to read the book to get there though - just reading a summary of what Nietzsche is about will probably be enough.
Linear Algebra, literally a new dimension.
Basically every humanities scholar complains about the complexity of the issue, like social of political, just because of them not knowing linear algebra as a tool. You can solve almost all problem using matrix and tensor.
Princeton Companion to Mathematics. Having such an encyclopedic volume at hand would have been great as a child. Very clear exposition, and breadth of content, from very relevant people in the field.
Still waiting for the Physics one, though!
Understanding Context, by Andrew Hinton.
Not directly a textbook per se, but was a deeply educational book for me about designing digital spaces and thinking about interface metaphors from a real-world, architecture-driven perspective.
The logic of failure, but it’s not a textbook.
Farnham Street blogs as a pdf
I fell in love with philosophy during a class where the textbook was "Gödel, Escher, Bach." The book explores the interconnectedness of mathematics, art, and music, and delves into consciousness, self-reference, and complexity. It introduced me to Gödel's theorems, Escher's illusions, and Bach's compositions. This sparked a deep passion for philosophical inquiry, leading me to further explore the mysteries of existence.