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 A curated list of awesome mathematics resources.
 
+All resources are freely available except those with a 💲 icon.
+
 # Contents
 
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 * [ProofWiki](https://proofwiki.org/wiki/Main_Page)
 * [Wolfram Mathworld](http://mathworld.wolfram.com/)
 * [The On-Line Encyclopedia of Integer Sequences](https://oeis.org) - Great compendium of many different integer sequences. Founded 1964 by N. J. A. Sloane.
+* 💲 [The Princeton Companion to Mathematics](https://press.princeton.edu/books/hardcover/9780691118802/the-princeton-companion-to-mathematics) - Timothy Gowers (Professor, Fields medallist), June Barrow-Green (Professor), and Imre Leader (Professor).
+* 💲 [Encyclopedia of Distances (4th Edition)](https://link.springer.com/book/10.1007/978-3-662-52844-0) - Michel Marie Deza, Elena Deza.
 
 ## Books
 
@@ -195,6 +199,7 @@ A curated list of awesome mathematics resources.
 ### Transition To Pure Rigour Math
 * 📝 [Basic Concepts of Mathematics](http://www.trillia.com/zakon1.html) - Elias Zakon
 * 📝 [Book of Proof](https://www.people.vcu.edu/~rhammack/BookOfProof/) - Richard Hammak (Virginia Commonwealth University)
+* 📖💲 [How to Prove It: A Structured Approach (3rd Edition)](https://www.cambridge.org/highereducation/books/how-to-prove-it/6D2965D625C6836CD4A785A2C843B3DA) - Daniel J. Velleman (Professor).
 
 ### Set Theory