Problem solved. It was my ignorance, nothing wrong with Windows or Wolfram software. The notebook cells were grouped. I was trying to open them with Shift-Enter. Instead I needed to select the cell or cells, then use the Cell->Grouping menu to open the selection.

Obviously, I am still a Mathematica newbie.

$Version
(* 12.0.0 for Mac OS X x86 (64-bit) (April 7, 2019) *)

This is what I see when I open the notebook, then open the first cell group and evaluate a few cells. What happens when you open the first cell group and evaluate some cells?

In a text editor I see the following starting at line number 11850. I assume the "data bytes" you refer to starts at line number 11919. That is indeed compressed image data that is assigned to the symbol Peppers. On line 11916: RowBox[{"Peppers", "=",. On line 11901: " Determining the number of red pixels in an image. (Figure 18.2).". Do you see the same or different?

I just downloaded that .nb and opened it in Mathematica 12.0 under macOS. No trouble at all evaluating input cells — no need to make cells Evaluatable.

Did you perhaps make some change to global settings in Mathematica itself?

Are you aware that there is actually a 2nd edition of the book? Presumably the freely-available, downloadable, Mathematica notebooks are coordinated with this 2nd edition rather than the original, 1st, edition.

[Ordering books from Amazon can be tricky, due to various formats and editions, including, sometimes cheap (and sometimes unauthorized) knock-offs.]

I am running version 12. I have a license for one workstation and one home. Both fail the same way and have been that way since I started with version 11.2.

I have no trouble download the .nb file onto a Windows 10 PC. The size on disk is three megabytes.

After opening with a raw text editor, I can see the Wolfram code that produces the table of contents, but most of the rest was data bytes like you would see in an encrypted or image file.