Ian Stewart's Hoard of Mathematical Treasures is the successor of Cabinet of Mathematical Curiosities by the same author. A cynic might suspect that it will contain leftovers for which there was no more space in Cabinet, but that is certainly not the case. There are fewer well known topics than in Cabinet, but that makes the book only more interesting.
There are more than 150 entries. The shortest consists of only one sentence (Halloween=Christmas), but the longest extends over eight pages. They are not all equally interesting, but Stewart writes in a very engaging way and can make even complicated things understandable. There are entries on the history of mathematics (e.g, the abacus, the equal sign, Egyptian fractions, the slide ruler, Hilbert's problems, the symbol for pi, the factorial symbol, and the square root symbol), entries on number theory (e.g., the rule of eleven, the Catalan conjecture, congruent numbers, the Green-Tao theorem, Euler's conjecture, and primes), entries on topology (e.g., hexaflexagons, flexible polyhedrons, the bellow's conjecture, the hairy ball theorem, horned spheres, knots, the ham sandwich theorem, the four color theorem, and how to turn a sphere inside out), and entries on applications of mathematics (e.g., codes and CAT scans). There are also quite a few entries that are really about physics (e.g., falling cats, antimatter, celestial resonance, global warming, and Lagrange points), which I found less interesting. There are only a few entries on well knows topics (e.g, magic squares and the Klein bottle) and I found only one entry copied from one of his other books (Common knowledge from Math Hysteria). For readers that want to become active there are many puzzles. Some are quite simple, others will keep you occupied for many hours. No less than 67 pages are devoted to the solutions.
Personally I like How to Cut a Cake and Math Hysteria better, because these book delve deeper in the problems that they discuss, but I still highly recommend Hoard of Mathematical Treasures.
