Signs are everything. Language is a system of signs and the more one examines this system, the more apparent it becomes that its got something of a life of its own. And any picture or sound or object or anything can be used as sign in context.
There is a very readable six-part history of modern signs written by Julia Turner for the blog, Slate (brought to our attention by Boing Boing). Among other things, Turner covers the the war over exit signs, personal cartography, “Legible London” (a TfL initiative) and the potential impact of GPS on road signs.