Originally Posted by dgatwood
Depends. If you plan to convert to Kindle, they pretty much require that you include one:
"Amazon strongly recommends the use of an HTML TOC for all books that would benefit from this navigation feature. This applies to most books, with the exception of fixed-layout children's books (see section 4) and fixed-layout graphic novels/manga/comics (see section 5)." (Kindle Publishing Guidelines, p.14.)
Barnes and Noble seems to suggest that it is a good idea:
"It is helpful to have a Contents Page in the eBook, even if there is not one present in the print version. The ebook Contents Page is linked to the individual chapters and helps with ebook navigation." (PubIt! ePub Formatting Guide, p.4.)
Apple's guidelines don't mention an HTML table of contents at all.
My advice would be to decide whether people are likely to access your content nonlinearly. If it's a fiction book, the answer is usually "no". If it is anything else, the answer is usually "yes".
Either way, include it, but if you don't think readers will actually use it, set linear="no" or put it at the end of the book. That way, kindlegen sees it and can use it to make navigation better on Kindle devices (which AFAIK don't support NCX navigation beyond next-chapter/previous-chapter jumping).
Also note that even on Kindle devices that provide an actual TOC-like representation, the NCX view is single-level. If you have a multi-level TOC, you might want to consider making your HTML TOC be linear.
Then HTML ToC is needed in the ePub when the ePub is going to be converted to Kindle eBooks. But other then that, you don't need one. Have it for the ePub going to Amazon, but not in the ePub that's for sale.
As for Barnes & Noble, they suggest a ToC and in that case, the NCX ToC is what should be used. B&N does not say there should be an HTML ToC. B&N sell ePub. ePub's best ToC is the NCX ToC.
As for Apple, the NCX ToC is all you need there too.