An indexer is a person who creates a book contents listing, also known as an index. An indexer is responsible for organizing and categorizing the topics and subtopics of a book, and then creating a list of page numbers where each topic can be found. This allows readers to quickly find information within a book. Indexers must have a strong understanding of the book's content and be able to identify key concepts and terms. They must also be able to write concisely and accurately.