An index at the back of a book helps readers find the information contained in a book. It differs in two ways from a table of contents, which is located at the beginning of a book: first, in an index the text can be analysed in a much more detailed way, and second, since its headings are sorted alphabetically,1 they are easier to find. As a result, by consulting an index, it is easier to find the pages where particular subjects, ideas, names, and places are addressed. While a table of contents shows the structure of the whole text, an index deals with its details.
I index non-fiction books, mostly (but not exclusively) from biblical studies. Using the final proofs (PDF file) as a basis, I read through the text, and in a separate file I create the index headings. When done, I edit, format, and apply cross-references where necessary. The result is a finished index (a file in “.docx” or “.indd” format) that the typesetter can insert into the typeset files.
On average, the length of an index for a non-fiction book is about 4 to 7 per cent of the number of indexable2 pages. For example, a book with 260 indexable pages will have an index of roughly 11 to 19 pages. Creating an index may require a few days and as much as a few weeks, depending on the book’s type and length.
For examples, see here.
1 Not all indexes are sorted alphabetically: in extensive scripture indexes, for example, entries are sorted canonically.
2 Indexable pages are all text pages of a non-fiction book except for the front matter and the bibliography.