It’s advisable, some would say mandatory, for an author to create a table of contents (TOC) for any work of nonfiction, or any book that is a collection of materials, like poetry, short stories, plays etc. For novels delivered in print, a table of contents is largely a waste of time and can be counterproductive. The reader is interested in following the flow of the story and does not want to see a table of contents for a host of reasons. For example, can you imagine buying a copy of Mario Puzo’s The Godfather and consulting the table of contents? Let’s see now, Chapter 22, “Michael Corleone Wipes Out The Heads of the Five Families.” That would kind of spoil the reading experience, wouldn’t it? Or, how about a great mystery novel with a TOC that reads, “Chapter 30: The Butler Confesses.” Again, TOCs and novels don’t mix. The one exception to this is if your book is in eBook format. Here, readers prefer a table of contents tied to chapter names because it helps them navigate the eBook.
On the other hand, if you’re writing a book on botany, you definitely want the reader to be able to have a quick reference guide to the contents of the book. If, for instance, she is interested in hybrid orchids, and you have a very detailed chapter on the subject, having a table of contents will obviously be a strong selling point. Some authors of non-fiction, such as biographers, might resist taking the time to include a TOC, but this is a mistake and can make your book less marketable.
Unlike an index, or even a glossary, a TOC is relatively easy to do. It’s no more than a compilation and listing of the parts of the book as you’ve defined them, sections, chapters, sub-chapters, for example. If it’s difficult to divide your book thusly, then you have a problem with structure and organization that needs to be solved before you do your TOC. But since books have pretty standard formats depending on genre (textbook, biography, anthology etc.), this kind of structure should not pose problems. Writing a TOC could easily be described as a POC, as in piece of cake.