"Four years ago, when the first edition of this book was written, information retrieval was beginning to crystallize out as a unified discipline. The process has gone further today. Several other books... have also offered a general survey, although each has contributed its own special emphasis. Many conferences on the subject have been held, and a constant stream of new articles has appeared, both in documentation journals and in those in the data processing field. Information retrieval is now recognized as a discipline, and further advances in theory are being made, What I described in the first edition as the key operation in retrieval — the subject description of documents — is being explored theoretically and experimentally, although we are still a long way from reducing this operation to rule (Chapter 3). There has been less new work on the design of descriptor languages, although ideas on the display of descriptor relations through thesauri and 'semantic maps' have been developed (Chapter 4). Access to files has been examined, particularly by those experienced in data processing."