Anyway I clicked through to The New Martin Gardner Mathematical Library to discover that it is exactly what I thought it might be, i.e. an updated version of his Mathematical Games column. The library is described thus:
The books based on Martin Gardner's enormously popular Scientific American columns and puzzles continue to challenge and fascinate readers. In these new editions, the author, in consultation with experts, has written updates to all the chapters, including new game variations, new mathematical proofs, and connections to recent developments and discoveries. New diagrams and illustrations have been added and old ones improved, and the bibliographies have been greatly expanded throughout.The web page looks unfinished, but it gives at least some of the titles that will be in the library, which I list below with links that I have added for convenience:
1. Hexaflexagons, Probability Paradoxes [I have linked to the Monty Hall problem as an example of this genre], and the Tower of Hanoi
2. Origami, Eleusis, and the Soma Cube
3. Sphere Packing, Lewis Carroll, and Reversi
It looks like the sort of good stuff that will provoke those familiar mental gymnastics of yore.