Some great books at high school level (listed approximately in the reading order):
H. Rademacher & O. Toeplitz (1967). The Enjoyment of Math. Princeton University Press.
J. R. Pierce (1980). An Introduction to Information Theory: Symbols, Signals and Noise. Dover Publications, second edn.
J. R. Weeks (2001). The Shape of Space. CRC Press, second edn.
H. S. M. Coxeter & S. L. Greitzer (1967). Geometry Revisited. The Mathematical Association of America.
D. R. Hofstadter (1999). Gödel, Escher, Bach: An Eternal Golden Braid. Basic Books, 20 anv edn.
R. Courant & H. Robbins (1996). What Is Mathematics? An Elementary Approach to Ideas and Methods. Oxford University Press, USA, second edn.
F. W. Lawvere & S. H. Schanuel (1991). Conceptual Mathematics: A First Introduction to Categories. Buffalo Workshop Press, Buffalo, NY, USA.
Some great books at high school level (listed approximately in the reading order):
H. Rademacher & O. Toeplitz (1967). The Enjoyment of Math. Princeton University Press.
J. R. Pierce (1980). An Introduction to Information Theory: Symbols, Signals and Noise. Dover Publications, second edn.
J. R. Weeks (2001). The Shape of Space. CRC Press, second edn.
H. S. M. Coxeter & S. L. Greitzer (1967). Geometry Revisited. The Mathematical Association of America.
D. R. Hofstadter (1999). Gödel, Escher, Bach: An Eternal Golden Braid. Basic Books, 20 anv edn.
R. Courant & H. Robbins (1996). What Is Mathematics? An Elementary Approach to Ideas and Methods. Oxford University Press, USA, second edn.
F. W. Lawvere & S. H. Schanuel (1991). Conceptual Mathematics: A First Introduction to Categories. Buffalo Workshop Press, Buffalo, NY, USA.