Hi... Math Undergrad here... here are some of the books that I started looking at in high school... none of them are too difficult for the high school level...
Excursions in Number Theory
C. Stanley Ogilvy and John T. Anderson
Dover: ISBN 0-486-25778-9
Introduction to Probability
John E. Freund
Dover: ISBN 0-486-67549-1
Q.E.D. Beauty in Mathematical Proof (This is a VERY fun book, high school students should really like it)
Burkhard Polster
Walker: ISBN 0-8027-1431-5
And finally, one that doesn't seem appropriate but only fits because of the lack of algebra and technical language...
Game Theory: A Very Short Introduction
Ken Binmore
Oxford: ISBN 0-19-921846-2
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Hi ... Math Undergrad here ... here are some of the books that I started looking at in high school ... none of them are too difficult for the high school level...
Excursions in Number Theory
C. Stanley Ogilvy and John T. Anderson
Dover: ISBN 0-486-25778-9
Introduction to Probability
John E. Freund
Dover: ISBN 0-486-67549-1
Q.E.D. Beauty in Mathematical Proof (This is a VERY fun book, high school students should really like it)
Burkhard Polster
Walker: ISBN 0-8027-1431-5
And finally, one that doesn't seem appropriate but only fits because of the lack of algebra and technical language...
Game Theory: A Very Short Introduction
Ken Binmore
Oxford: ISBN 0-19-921846-2