Because this is an EPUB 3.0 document, it has both a NAV file (newly defined in EPUB 3)
and an NCX file (required for backward compatibility with EPUB 2).
Both the Nav file (bk01-toc.html) and the navmap hierarchy in the NCX file (toc.ncx)
cover the Chapter and Section levels.
The spine entry lists the enclosed content files, in the "normal" reading order.
The toc attribute's value (ncx) tells us to look for toc.ncx,
a file which contains EPUB 2 navigation hints and other metadata.
The guide entry lists only one reference:
The cover page (Cover) is "cover.html".
The manifest entry for item toc (in the OEBPS/content.opf file)
indicates that it is the Nav file: properties="nav".
toc.xhtml is an XHTML file,
which guarantees some useful syntactic decorum (e.g., closing tags).
The NCX Navigation Center eXtended file (toc.ncx) is included,
as required for backward compatibility with EPUB 2.