Okay, at http://tex.stackexchange.com/questions/358996/fontspec-warning-tu-clash I found that I needed to run sudo fmtutil-sys --all. That resolved the fontspec clash, but I have a new error:

kpathsea:make_tex: Invalid fontname `[fonts/PTSansNarrow-Regular.ttf.ttf]/OT', contains '['

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!
! fontspec error: "font-not-found"
! 
! The font "PTSansNarrow-Regular.ttf" cannot be found.
! 
! See the fontspec documentation for further information.
! 
! For immediate help type H <return>.
!...............................................  
                                                  
l.30 {PTSansNarrow.ttf}

For reference, PTSansNarrow-Regular.ttf is very much in my fonts directory. 😢

@pydanny hmm.. have you tried this using TeXMaker? The log/error output is generally informative and helpful.
The build options for XeLaTeX work for me. As does the quick build for DVI->PS.
Modifying the options via Option=>Configure Texmaker=>(Commands and Quick Build) may prove useful.
Wish I could be more helpful.

@clayball, I'll give TeXMaker a try!

I think it is adding an extension for you: note PTSansNarrow-Regular.ttf.ttf is what it was searching for.

What is your current font declaration?

@pydanny, best of luck with everything!

Also, I should mention that I cannot reproduce the initial error. My guess is it's related to your current font declaration.

Strangely when I look at the PT Sans Narrow font on my comp, it comes in a .ttc font collection rather than an individual file. Had you generated them manually?

@mpacer, the font declaration causing the problem is:

\setsansfont [Path = fonts/,
    UprightFont = PTSansNarrow-Regular.ttf,
    BoldFont = OpenSans-Bold.ttf,
    ItalicFont = AvenirNextCondensed-Italic.ttf
 ]
{PTSansNarrow.ttf}

The font is stored in a fonts directory. It's also in my Mac's fontbook utility. Not sure if there's a conflict.

However, what've discovered though is that when I comment out this command, the book compiles without a problem. In the rendered book's section on typographical conventions, everything looks fine. HOORAY!

So I think the problem is resolved. However, since the book is 527 pages, it might be a while before we discover any problems.

Does get you a functional font declaration? My guess is that if you don't pass an explicit Extension parameter it defaults to Extension = .ttf which seems backwards for xelatex given it's otf support, but 🤷‍♀️.

\setsansfont [Path = fonts/,
Extension = .ttf,
UprightFont = PTSansNarrow-Regular,
BoldFont = OpenSans-Bold,
ItalicFont = AvenirNextCondensed-Italic
]
{PTSansNarrow}