What is mathematical symmetry?
Symmetry in mathematics
Hermann Weyl, Symmetry. Reprint of the 1952 original. Princeton Science Library. Princeton University Press, Princeton, NJ, 1989. viii+168 pp. ISBN0-691-02374-3
Mark Ronan, Symmetry and the Monster, Oxford University Press, 2006. ISBN978-0-19-280723-6 (Concise introduction for lay reader)
Marcus du Sautoy, Finding Moonshine: a Mathematician's Journey through Symmetry, Fourth Estate, 2009