EPUB books are sold by Apple, Kobo, Barnes & Noble, Sony, and others.
Though each of these ebook types is also sold by various small, independent retailers, the companies that I mentioned are the ones that make up the bulk of ebook sales worldwide.
There are two Table of Contents types that are present in ebooks:
1.) the first type is the separate, not-part-of-the-text Table of Contents (also known as an “NCX Table of Contents”), and it is the most ubiquitous. All properly-formatted ebooks have one (even semi-properly-formatted ones do), whether they are MOBI books (where it is normally invisible and often hard to get to) or EPUB books (where it is also normally invisible but is very easy to get to).
In EPUB books this Table of Contents type becomes visible when the reader clicks on the “Table of Contents” button. It is displayed as a single scrolling list of chapters on which you can click and be taken to the corresponding section of the book. (I’ll speak about how Kindle devices handle this type in a moment.)
2.) the second type is the visible-and-clickable-within-the-text Table of Contents (also known as an “HTML Table of Contents”), and it is the one that is most useful for MOBI books though (unfortunately) can be the most difficult to create and place within a book. (There’s an easy way around this that I’ll speak about in another post.)
Unlike the NCX type (which on a Kindle device or app is often difficult or even impossible to get to), an HTML Table of Contents is almost always visible within the running text and is displayed just like any other page of the book, except that it is composed entirely of clickable links that correspond to the various sections of the book. It is most often found right near the beginning of the book or at the very end, and can usually be gotten to merely (though sometimes tediously) by turning enough pages until you arrive. It can be as short as a single page or, in books with many chapters, several pages long.
A Kindle device will sense its presence in the book and will make it more easily available by listing it in in the book’s “Go to…” list. The “Go to…” list is a handy little window which, with a couple of clicks, pops up and gives the reader a list of places to “Go to” within the book. A complete “Go to…” list consists of six items: table of contents, beginning, page, cover, end, and location. Without an HTML Table of Contents, the “table of contents” item in the “Go to…” list will be grayed out and un-clickable. Without a clickable Table of Contents, getting to a particular spot within an ebook can get very tedious very quickly.
A Kindle book’s NCX Table of Contents is how the device keeps track of four of the other “Go to” items: the beginning, the cover, the end, and any specific location within the text that you might want to go to. People have a tough time specifying a “location” with a Kindle book because “location” to a Kindle does not correspond to any particular page or chapter. It has something to do with approximately how many kilobytes of data you are from the beginning of the book, so it’s not generally very useful to people. (Aren’t you glad that you asked?) It’s useful to the machine, though (and, thus, in this case here to you), because it’s this “knowing the distance from the front of the book” that enables the Kindle to place your bookmarks into the text.
The NCX also keeps track of the beginning of each chapter (even though the device often won’t show you the list). This makes it possible on certain Kindle devices (the Kindle Keyboard is one) to, with just a click, jump to the start of the next chapter. This is certainly easier than going page-by-page but is still no cakewalk if you want to get to Chapter 43. That’s why an HTML Table of Contents is so important in a Kindle book.
The two places where I’m sure that a Kindle book’s NCX Table of contents will be visible are the Kindle for Mac/PC app and the downloadable Kindle Previewer, which is used to test a Kindle book before it’s published. Using either of those apps, if you click on the button for the HTML Table of Contents for a book that doesn’t have one, you will get an error message stating that the Table of Contents is “missing” (as though the HTML Table of Contents is the only one). On the other hand, if you click on the button for the NCX one, it will be quite visible and clickable. I’m not sure if any of the actual Kindle devices will show the NCX Table of Contents. If not, you’re left with only the “Go to” items again. (By the way: if you click on the NCX button and are told that even that one is missing, the formatting for that book is really messed up and will be very difficult for a reader to navigate.)
The last “Go to” item, the page, is visible in those Kindle books that have been specially coded to have actual page counts that correspond to what the page number would be if it were a printed book. Some people have thought for a long time that this might be useful, so Amazon made this coding ability available not too long ago for those programmers who want to take advantage of it. Personally, I don’t.
A book’s “Go to…” list is especially useful when the Table of Contents is at the end of the book. Why would you want the Table of Contents there? Because that way it takes no space away from the book’s sample pages, which, for books that are for sale on Amazon, consist of the first ten per cent of the book. You want the sample to be as useful and enticing as possible to a potential customer, something that’s difficult when a good portion of the sample is several pages of nothing but “Chapter 1,” “Chapter 2,”… “Chapter 43,” etc. If, on the other hand, your chapter titles are interesting and descriptive (“Chapter 1: How I Got Thin in 30 Days”) then, yeah, keep the Table if Contents in the front, where it will be seen as part of the sample.
Interestingly, though Kindle books should have both Table of Contents types, many times they do not. Amazon supposedly requires it (that requirement is, in fact, stated in its Publishing Guidelines) but they do not seem to enforce that requirement in actual practice. That means that many Kindle books (especially self-published ones) have only an NCX Table of Contents, which, for the reasons stated above, can be a real drag. EPUB books, on the other hand, need only the NCX type. An HTML type is superfluous since, with a click or two, an EPUB book’s NCX Table of Contents pops up for you to see and click on, and doesn’t even take pages away from the book’s sample.
I hope that this helps. How a Table of Contents for your book is actually created I will discuss in another post. Leave a comment with any questions, concerns, or suggestions for future topics.