Does Your Book Need an Index?
If your book is a work of fiction then no, it does not need an index. Many people believe that a fictional work has too much reader interpretation to be locked into the concrete structure of an index. (Lancaster, in his book Indexing and Abstracting Theory and Practice, has a chapter that tackles the challenges of indexing imaginative works, such as fiction or film.)
Some works of fiction are indexed, but these are generally classics of literature. There are exceptions, of course, and those indexes tend to be a delight to read (for those of us who read indexes).
If your book is nonfiction then yes, it needs an index. Your book undoubtedly contains a great many threads of information woven throughout the text that need to be gathered into index entries. An index ensures that readers who refer to your book will find the particular information that they seek.
Here are some articles supporting my contention that a nonfiction book without an index is like a town without addresses: