All the Mathematics You Missed
That is, a book that covers the motivation and important ideas of various fields of mathematics at the graduate/professional level (Measure Theory, Functional Analysis, Representation Theory, Algebraic Topology, Stochastic Analysis, Harmonic Analysis, Number Theory etc.)? I know there's the Princeton companion, but it is a massive book and not exactly suitable for light reading while commuting say.
Thank you.
You might have a look at Zeidler: Quantum Field Theory I and see whether you like that. There are also two more volumes. In many parts of the books there a great explanations from a motivational point of view, for instance on cohomology.