[saityi]

Introduction to Graph Theory - Richard J. Trudeau

A History of Pi - Petr Beckmann

Journey through Genius: The Great Theorems of Mathematics - William Dunham

How to Bake Pi - Eugenia Chang

These are all sort of 'pop-math' books -- that is, they're more intended to spark a joy & love for math than teach rigorous mathematics. Great Theorems and A History of Pi include a lot of history (edit to add: in addition to covering the math involved!) -- did you know some mathematicians in history would duel over their theorems? That theorems were a carefully guarded secret instead of something you shared?

Introduction to Graph Theory is specifically intended as an introduction to mathematics for 'the mathematically traumatized'.

In my opinion, after reading these, if you've sparked a joy for the puzzles and fun of mathematics, then I would then suggest branching out into more formal presentations of them relevant to your interests... it's much easier to slog through a book on abstract mathematics when you receive from enjoyment from the puzzles presented.