Symbol Standards

Symbol Design and Testing

IEC 80416-1:2001—Basic principles for graphical symbols for use on equipment

Symbol Sourcebook: An Authoritative Guide to International Graphic Symbols: Dreyfuss, Henry. New York: John Wiley & Sons. 1984
Symbol Signs: The complete study of passenger/pedestrian-oriented symbols developed by The American Institute of Graphic Arts for the U.S. Department of Transportation: This book captures enough of the design and selection process that the AIGA and U.S. DOT went through in arriving at the final symbol list, that you can learn some of the key issues that are important. Things like different interpretations of shapes by different cultures, implications of a crossed knife and fork (prohibition), and the problems in designing airport arrival and departure symbols.
Graphical-Symbols.info: At $500 for the initial buy-in, and $150/year thereafter, this may seem a steep price, but it is an invaluable (if sometimes cumbersome) tool. This database contains the entire current set of ISO and IEC symbols for use on equipment, with most downloadable in at least PDF format.
Official Signs & Icons 2: For $250, this is perhaps the most comprehensive and accurate symbol clipart library around. It is the official distribution source for several symbol sets, although it provides no background information about what exactly each symbol is supposed to be used for. For that, you need the original source documents, such as the IEC and ISO standards.