Wow! How much time do you have? Some of the work is surprisingly accessible. I can verify that I am mortal but read large chunks of 'Theory of Games' and (just about) understood it. This was really important to do for the book because actually most people stopped reading the book and relied on textbooks written afterwards that were based on it. His game theoretic treatment of poker is a classic and I was charmed by how 'bluffing' magically appeared as a strategy when he did the maths.

Mathematical foundations of QM is still important today but it depends how deep you want to get into that. 'The Computer and the Brain' is very readable--and astonishing when you think that it was written in 1956. It feels very contemporary in some ways.