Any of Smullyan's books, particularly "What Is The Name Of This Book?", "The Lady Or The Tiger", "Alice In Puzzleland". Lots of fun, and not what high school students would consider math. "Disguised" as mere logic puzzles, they are great for learning formal logic and ultimately introduces Godel's Incompleteness Theorems. Much easier and more fun than Godel, Escher, Bach (which is truly a wonderfully fantastic book, if you have the students who are ready for it).
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Any of Smullyan's books, particularly "What Is The Name Of This Book?", "The Lady Or The Tiger", "Alice In Puzzleland". Lots of fun, and not what high school students would consider math. "Disguised" as mere logic puzzles, they are great for learning formal logic and ultimately introduces Godel's Incompleteness Theorems. Much easier and more fun than Godel, Escher, Bach (which is truly a wonderfully fantastic book, if you have the students who are ready for it).