The Princeton Companion to Mathematics

This book by Timothy Gowers came up once on Lex Fridman in the episodes we index. That mention was a genuine on-air recommendation, not just a passing reference. The people who brought it up: Grant Sanderson. Every occurrence below links to the episode page and to the exact second it was said on YouTube.

This is a one-of-a-kind reference for anyone with a serious interest in mathematics. Edited by Timothy Gowers, a recipient of the Fields Medal, it presents nearly two hundred entries, written especially for this book by some of the world's leading mathematicians, that introduce basic mathematical tools and vocabulary; trace the development of modern mathematics; explain essential terms and concepts; examine core ideas in major areas of mathematics; describe the achievements of scores of famous mathematicians; explore the impact of mathematics on other disciplines such as biology, finance, and music--and much, much more.