I can't really think of a suitable one TBH; Python's completely out of the running, Java and C# have a lot of unnecessary (for this goal) boilerplate, Pascal is not a bad choice.
Maybe Javascript? The books can then instruct "type this into an HTML file".
In my mind, a more modern platform would be a simulated one that has its own machine language (byte-code compiled, perhaps) so that these books, which take you all the way into machine language, would make sense.
Not for the book type format - the kids will be typing the code in, not copying + pasting them.
Significant whitespace is a killer in printed form; so Python is not even in the running.
And page 23 teaches you about significant whitespace, and how to configure several text editors that a kid's likely to have available to actually show it like that. Heck, I use Panic's Nova for my text editing and it does that out of the box with no configuration needed.