There is a tradition in hackerdom of informally tagging important textbooks and standards documents with the dominant color of their covers or with some other conspicuous feature of the cover. Many of these are described in this lexicon under their own entries. See Aluminum Book, Camel Book, Cinderella Book, daemon book, Dragon Book, Orange Book, Purple Book, Wizard Book, and bible; see also rainbow series. Since about 1993 this tradition has gotten a boost from the popular O'Reilly and Associates line of technical books, which usually feature some kind of exotic animal on the cover and are often called by the name of that animal.