Reading

"The only thing that you absolutely have to know, is the location of the library." - Albert Einstein

Gödel, Escher, Bach: an Eternal Golden Braid by Douglas Hofstadter
A very lengthy book introducing various aspects of formal systems, the major theorems arising from Hilbert's Program, consciousness, as well as some interesting topics in Baroque music and art. So far so good!
Electricity and Magnetism by Edward M. Purcell & David J. Morin
I chose this textbook as opposed to Griffith's slightly more popular "Introduction to Electrodynamics" as it introduces magnetic fields as a consequence of Special Relativity, an exposition which I have thoroughly enjoyed so far. It also contains thorough and wordy explanations behind the consequences of the equations - very useful for developing a strong intuition.
Linear Algebra Done Right by Sheldon Axler
Eye-opening and rigorous treatment of abstract Linear Algebra, which has helped me develop deeper levels of understanding behind the key objects in the field. I'd recommend a well-motivated introduction first such as Gilbert Strang's "Introduction to Linear Algebra", as a high-level understanding helps massively with the intuition / maturity needed to work through Axler's book.
Data Analysis for Scientists and Engineers by Edward L. Robinson

The Character of Physical Law by Richard Feynman
Although presented in a rather "pop-sciencey" style, it contains interesting discussions about the scientific method and the overarching conservation / symmetry laws. I particularly enjoyed the "Distinction of Past and Future" chapter, which explains the irreversibility of macroscopic phenomena as a consequence of thermodynamics / entropy. "I think that we are very lucky to live in an age in which we’re still making discoveries. It’s an age which will never come again."
Surely You're Joking, Mr. Feynman! by Richard Feynman
Fun anecdotes highlighting Feynman's extraordinary character and inspiring me to make the most out of life.
Classical Mechanics by John R. Taylor
Great textbook covering topics from high-school in more detail such as conservative forces, oscillations, non-inertial frames as well as an excellent introduction into the Lagrangian & Hamiltonian formalisms.