Steinhaus, Mathematical Snapshots Polya, Mathematics and Plausible Reasoning Pappas, The Joy of Mathematics Gardner, The Colossal Book of Short Puzzles and Prob

Steinhaus, Mathematical Snapshots Polya, Mathematics and Plausible Reasoning Pappas, The Joy of Mathematics Gardner, The Colossal Book of Short Puzzles and Problems Winkler, Mathematical Puzzles: A Connoisseur's Collection Andrews, Number Theory (Dover Books on Advanced Mathematics) - The last book is not really that advanced, It can be understood by most with an understanding through high school algebra II.