There is only one file type that uses the .NCX extension:
EPUB Navigation Control XML File
Table of contents file created according to the XML-based Open Packaging Format (OPF) standard, which is part of the EPUB eBook specification; stores the document title and author, as well as the navigation points for the eBook; used as part of an overall .EPUB archive.
According to the Open Packaging Format 2.0 specification, NCX files should be created with a corresponding .OPF file, which contains the linear reading order and other eBook metadata.
NOTE: Since NCX files are saved in an XML format, they can be viewed and edited with a text editor.
Updated: September 8, 2010