I really doubt that you will learn much about the ZenCart overrides in the css book.

A good purchase though... as long as the book that you purchased does not dwell on things that were popular in the past and are becoming passe today. Don't spend a lot of time on tables, frames, inline frames etc, many more items. Try to glean as much as you can about the tags like div, li, ul, definitions and other generic things.

Back to the overrides: You really don't have to know the code that allows this to work, at least at this time. You are like me probably, too anal and analytical and conclude that to understand that you must know how this is happening, Yes at least my first 2 months trying to work with this cart. Just grasp the concepts that are the underpinnings of this cart and work within them at first anyway. Probably, a good 80% of the functional files in the catalog section are overrideable, meaning that if they define an section that you do not want or want to add you can make your custom edits to them.

Now one can edit the actual file and things will be ducky right up until there is an upgrade or patch. then try to remember all the files that you have altered...
Ergo the override system. Your edited files reside in folders that you have created to house only your edited files and a patch or upgrade does not overwrite these directories. That is basically all that the override system exists for. Not a complicated plot to drive you crazy.

With 1.3.x and the template_default as a base, there are very few files that you must even touch to have a fully functional cart that can have your "special" look attached to it.

there is a combination of stylesheet, the admin and specific php files that control all of the representation (placement & styling) of the elements rendered. Most all can be cotrolled by the stlye sheet and in the admin. Only when this can not produce the result should one even look to the .php files.

Take two steps back, a deep breath, and just try to use what is provided first before embarking on a quest for the 'holy grail'.

"Customisation" by the way, there are Brits here and there.