The Princeton Companion to Mathematics

Amazingly coherent to understand. Although print is too small.Takes time to read.

Brought both volumes. I am engineer working in ocean industry, background hydrodynamics, and steel structures. Mathematical background is important for reading research papers and books in my own field. To do that intuitive mathematical understanding is important.

This is one of those reference books that you feel like reading for pleasure as well as information. It is unlike an encyclopedia. More like a list of well researched blog posts organized by category.