Essential Reading for the "Apprenti Mathematicien"
High scool level
Courant, Richard; Robbins, Herbert What is mathematics? An elementary approach to ideas and methods. Oxford University Press, New York, 1979
Rademacher, Hans; Toeplitz, Otto The enjoyment of math. Translated from the second (1933) German edition and with additional chapters by H. Zuckerman. Princeton Science Library. Princeton University Press, Princeton, NJ, 1994. iv+205 pp.
W.Sierpinski A selection of problems in the theory of numbers.
Translated from the Polish by A. Sharma. A Pergamon Press Book The Macmillan Co., New York 1964 126 pp.
Undergraduate level
Hilbert, D.; Cohn-Vossen, S. Geometry and the imagination. Translated by P. Nemenyi. Chelsea Publishing Company, New York, N. Y., 1952. ix+357 pp.
Chinn, W. G.; Steenrod, N. E. First concepts of topology. The geometry of mappings of segments, curves, circles, and disks. New Mathematical Library, Vol. 18 Random House, New York; The L. W. Singer Co., Syracuse, N.Y. 1966 viii+160 pp.
Prasolov, V. V. Intuitive topology. Translated from the Russian manuscript by A. Sossinsky [A. B. Sosinskii]. Mathematical World, 4. American Mathematical Society, Providence, RI, 1995. viii+95 pp.
Milnor, John W. Topology from the differentiable viewpoint. Based on notes by David W. Weaver. Revised reprint of the 1965 original. Princeton Landmarks in Mathematics. Princeton University Press, Princeton, NJ, 1997. xii+64 pp.
Graduate level
Milnor, J. Morse theory. Based on lecture notes by M. Spivak and R. Wells. Annals of Mathematics Studies, No. 51 Princeton University Press, Princeton, N.J. 1963 vi+153 pp.
Milnor, John W.; Stasheff, James D. Characteristic classes. Annals of Mathematics Studies, No. 76. Princeton University Press, Princeton, N. J.; University of Tokyo Press, Tokyo, 1974. vii+331 pp.
How to write a mathematics paper?
A good selection of ressources on this topic can be found
here,