As a somewhat lapsed mathematician, I enjoy reading popular mathematics books. This series by Ian Stewart is a particular gem: a miscellany of mathematical facts connected by a parody of Sherlock Holmes.
The puzzles were all the right sort of difficulty for me to enjoy: I had to sit, think and occasionally doodle, but they were all solvable. However I remain sadly too aware of my limitations to think that I could possibly prove the ABC conjecture, or other unsolved issues that Stewart mentions as he brings modern mathematics into the mix.
If you are of a mathematical inclination, have a fondness for puzzle-solving and like Sherlock spin-offs, then this is a fun book with bits to learn.