This is a landmark book. For anyone interested in language, in dictionaries and thesauri, or natural language processing, the introduction, Chapters 14, and Chapter 16 are must reading. (Select other chapters according to your special interests; see the chapter-by-chapter review). These chapters provide a thorough introduction to the preeminent electronic lexical database of today in terms of accessibility and usage in a wide range of applications. But what does that have to do with digital libraries? Natural language processing is essential for dealing efficiently with the large quantities of text now available online: fact extraction and summarization, automated indexing and text categorization, and machine translation. Another essential function is helping the user with query formulation through synonym relationships between words and hierarchical and other relationships between concepts. WordNet supports both of these functions and thus deserves careful study by the digital library community.