I might add the following words about the NCX from the current Amazon Kindle Publishing Guidelines:
3.3.1 TOC Guideline #1: Logical TOC (NCX) Is Recommended
Amazon strongly recommends that all Kindle books include both logical and HTML TOCs. The logical TOC is very important for a good reading experience, because it allows a reader to navigate between chapters easily. Users expect to see an HTML TOC when paging through a book from the beginning, while the logical TOC is an additional way for users to navigate books. The inclusion of a logical TOC is especially important for books that are longer than 20 pages.
Logical TOCs are generated using a navigational control file for XML application (NCX). Creating an NCX exposes the hierarchical structure of a Kindle book and allows the user to navigate through it.
In NCX-enabled books, users can see where they are in the book because the part, chapter, or section is exposed. This progress indicator also shows relative progress through the book.
Logical TOCs are part of the IDPF 2.0 specification