I have 200€ to spend on books. Which essential and fundamental books should I get, in either Computer Science(C/C++/Python) , Physics (Quantum Physics, Nuclear physics, Electronic Engineering ) and Mathematics (statistics, stochastic calculus, finance, logic)? Must-know topics and fields and their relative must-read books are also accepted even if not within the choices above cited.

I was planning on buying "Probability: Theory and Examples" By Rick Durrett but considering the price I need to think about it and know whether it's a book worth buying or not.