The index of a non-fiction book is by far the most important part of the book. This is because the non-fiction book serves most of its life as a reference tool used to look up topics. Thus, the index is a search-engine used to find specific and arcane topics on an ad-hoc basis.
I regard the index of a book to be a separate literary work in which the indexer should share in the credit for the work. Indexing requires specialized skills of organizing information. The point of an index is that it must actually be useful for finding topics. Thus, the indexer must understand and master the text of the book in order to create an index of the book.
Here are two indexes I have created: