1. P. G. de Gennes, The Physics of Liquid Crystals, Oxford University Press, 1974, ch. 1.
2. Sydney Coleman, ``Secret Symmetry: an introduction to spontaneous symmetry breakdown and gauge fields'', in Aspects of Symmetry, Selected Erice Lectures, Cambridge University Press, Cambridge, 1985, p. 113, section 1.
3. David Mermin, ``The Topological Theory of Defects in Ordered Media'', Reviews of Modern Physics 51, p. 591 (1979).
4. Dan L. Stein, ``Topology of Order Parameter Spaces of Condensed Matter Systems'', PhD Thesis, Princeton University, 1979.
Statistical Mechanics: Entropy, Order Parameters, and Complexity, now available at Oxford University Press (USA, Europe).