[KevinH]

Took a peek and the structure of that epub is very strange:

For example: cover.xhtml is in OEBPS/Text/
but nav.xhtmnl, copyright.xhtml and p01c01.xhtml are all at the root of the epub

And your single image (cover.jpg) is stored in OEBPS/OEBPS/

Your content.opf is inside of OEBPS while you toc.ncx is at the root of the epub

And you stylesheet "stylesheet.css" is at OEBPS/

In addition your files have xml headers but no DOCTYPEs

As long as the links among the files are right, none of this should make a difference, although I have rarely seen an epub with this mixed up of a file structure.

So I used Sigil tools to
- move all of the .xhtml into OEBPS/Text/
- the .css files into OEBPS/Styles/
- the .jpg files into OEBPS/Images/

And I moved the .opf and .ncx into OEBPS/

Ran all pages through Sigil's Mend and then save it as a new epub.

This epub showed up just fine in Books.app (iBooks on my Mac). So I assume Books must have some rules unwritten rules about structure.