The excellent back-of-the book index contains terms that navigate readers to find the indexed term on the important book page and related to the content of the book. The conceptual characteristic of indexed term is one of the difficulties that faced by the professional book indexers. They should ensure that the indexed term directs the reader to a book page with the relevant information so the readers can understand the meaning or information in the indexed term. This study aims to identify the relevance of the indexed term. We consider the relatedness of the text containing the indexed terms with the book domain. We calculate the semantic relatedness of the data set, using the WordNet lexical database. We have evaluated the performance of semantic-relatedness method on books from the knowledge domain of Biology, Religion and Computer Science using Kappa Statistic Values. The result implies that the relatedness to the knowledge domain of the book influences the relevance of an indexed term. The Kappa values that obtained from the results test show that this semantic relatedness approach is reliable and could contribute to identify the relevant indexed term.