Talking about the origin of symbols and their interpretations, the factors influencing the design of icons, symbol fonts, octicons: the github's new symbol font, and icons that we still use extensively but are not relevant anymore.
Charles Hartshorne, Paul Weiss & Arthur W Burks. Cambridge, MA: Harvard University Press. • Atkin, A. Peirce’s Theory of Signs. (2010). The Stanford Encyclopedia of Philosophy. Edward N. Zalta (ed.). Accessed December 29, 2012. • The Noun Project blog: http://thenounproject.tumblr.com • A List Apart: The era of symbol fonts • The github blog: https://github.com/blog/1135-the-making-of-octicons • Scott Hanselman: The Floppy Disk means Save, and 14 other old people Icons that don't make sense anymore