What's an NCX?  Why is that different than the "Table of Contents?"

If you've been researching eBooks at all, whether it's for MOBI (Amazon Kindle) or ePUB, you've probably heard about the "NCX."  You've probably seen people asking about it--how to make one for Amazon, or why it's different than the "TOC," meaning "Table of Contents."  If you're working solely in Word, trying to DIY your eBook, this can be a tough thing to create, and harder to understand. 

If your ePUB version book is viewed in Adobe Digital Editions, which is called “ADE,” yes, you will see a "Table of Contents" pane on the left-hand side. This allows your readers to “click around" to any part of the book that they desire.  In an ePUB, this is not a traditional, "typed" TOC; it's actually normally invisible. You can't page to it, by flipping through pages of an eBook.  It's created in a special file (the NCX), which is invisible until it's called to be displayed by a reading device or software (like Adobe Digital Editions, which is what you see in the image--that pane on the left is the NCX, being displayed for human viewing). The important part of an NCX is that it's not really for the people; it's for the device.  It's how the device knows where it is, in the book; what the "playorder" is (Chapter 1, 2, 3, etc.), and allows the human reader to navigate by clicking on any of the entries, to go wherever they want to go.

Other ePUB readers have different ways of showing the Table of Contents, or making it available to their readers. All are well-known to ePUB readers, and so this should not be something you need to worry about.  Below you can see an ePUB of "The Prince and the Pauper," by Mark Twain, displaying the "TOC ncx" on the left-hand side.  Those entries are clickable links, which enable the reader to navigate around the ebook at will.

“The Prince and The Pauper,” produced digitally by Ignacio Fernández Galván and has been used herein for illustrative purposes with his express permission.