Extensive essays on key topics in the field such as the organization of knowledge and information policy. Committe on Library Technology. ALA Glossary of Library and Information Science . Chicago, ALA, 1983. Although quite dated, this is still an essential source for historical terms and basic library terminology.
Entries are brief but clear.
1983 Harrod's Librarians' Glossary . Harrod's is a classic work which has been updated frequently. The current edition includes 9,000 terms for the field. Dictionary of Information Science and Technology . Boston, Academic Press, 1992.