Recently I've been browsing with great delight a recent book called Mathematical Puzzles: A Conoisseur's Collection, by Peter Winkler. These puzzles have been chosen because they're typically much more interesting than, say, the puzzles in the Monthly. They're aimed at people with a math background, but not necessarily an advanced one. Some of the solutions are fairly complex, but almost none require math beyond calculus (and very few go even that far). Besides a sections on probability, geometry, games, combinatorics, etc., there's also a non-mathy fun section on geography puzzles, a section of especially tough puzzles, and finally, a section of easy-to-state unsolved problems. Most math-funners have seen some of these problems before, but I'd guess at least 2/3 of them were new to me, and well worth the price ($18 on Amazon (paperback)). --Dan