Compact discs, which have revolutionized the recorded music industry, are doing the same for the book publishing industry. A number of reference books—such as trade directories and multivolume encyclopedias—are already available in compact-disc format. And now trade publishers are working out the legal issues involved in publishing their books in compact-disc format. How is the compact-disc technology applied to books? The heart of the system is the same as that used for recorded music. Any kind of information that can be digitized—converted to the numbers 0 and 1—can be transferred to the 4.7-in. diameter compact discs. Words and pictures, of course, are digitized in the common personal computer. Instead of outputting the digitized information exclusively as sound, the new technology hooks up a compact-disc player to a computer. The information stored on the disc is then output as words and pictures on the screen and as sound emitted through a speaker. Compact discs offer several important advantages over traditional delivery systems for printed information. First, compact discs can hold a tremendous amount of information. A 100-volume encyclopedia could fit on a single disc. The space storage advantages are considerable. Second, compact discs offer the accessing ease of an online system. If the user wants information on subatomic particles, he or she simply types in the phrase and the system finds every reference to the subject. On request, the citations or even the entries themselves can be printed out on paper. And third, information stored on compact discs can be updated much less expensively than paper information. The subscriber or purchaser simply receives an updated disc periodically.