The Princeton Companion to Applied Mathematics

The Princeton Companions to Mathematics and Applied Mathematics are beautiful to leaf through at the library. They're also hardcore heavy-weight (physically) and unlikely to be read twice, so don't buy them. My personal take is that good linear algebra books at any level are great "tours of mathematics". Start with Strang and never stop. In a few years you'll be balled up with Kreyszig scribbling proof attempts in receipts, flaming unkempt hair and everyone around you will think you're weird but you'll be so, so happy.

In terms of depth and breadth, the Princeton companions get close to Feynman.[1][2] A more formal approach appears in handbooks.[3][4] [1] Gowers et al., The Princeton Companion to Mathematics. https://press.princeton.edu/books/hardcover/9780691118802/th... (https://press.princeton.edu/books/hardcover/9780691118802/the-princeton-companion-to-mathematics) [2] Higham and Dennis, The Princeton Companion to Applied Mathematics. https://press.princeton.edu/books/hardcover/9780691150390/th... (https://press.princeton.edu/books/hardcover/9780691150390/the-princeton-companion-to-applied-mathematics) [3] Zwillinger, CRC Standard Mathematical Tables and Formulae. https://www.crcpress.com/CRC-Standard-Mathematical-Tables-an... (https://www.crcpress.com/CRC-Standard-Mathematical-Tables-and-Formulas/Zwillinger/p/book/9781498777803) [4] Bronshtein, Handbook of Mathematics. https://www.springer.com/gp/book/9783540721222