Ian Stewart is a very good mathematics popularizer and I understand why many of his books are in this list. But I really don't understand why none of Martin Gardner's books are listed here (at the time I'm writing this).

My favorite Martin Gardner book (actually, an updated collection of Scientific American Mathematical Games columns) is The Collosal Book of Mathematics. From the book's cover: "Number Theory, Algebra, Geometry, Probability, Topology, Game Theory, Infinity, and other topics of recreational mathematics." I would say its difficulty level is "+".

Ian Stewart is a very good mathematics popularizer and I understand why many of his books are in this list. But I really don't understand why none of Martin Gardner's books are listed here (at the time I'm writing this).

My favorite Martin Gardner book (actually, an updated collection of Scientific American Mathematical Games columns) is The Colossal Book of Mathematics. From the book's cover: "Number Theory, Algebra, Geometry, Probability, Topology, Game Theory, Infinity, and other topics of recreational mathematics." I would say its difficulty level is "+".