These tricks are usually done to increase speed or reduce space. For most (especially Microsoft) BASIC, constants are stored within a tokenized line as ASCII (as entered), and converted to a floating point number every time they are evaluated. This is a time consuming process. Assigning the number once to a variable to be used thereafter will skip this part ...
The answer by Raffzahn is very good, except that I disagree that ZX80/81 background is all that important and I also feel he missed one important trick.
I personally know most of these tricks from studying BASIC loaders for ZX Spectrum games. You see, yes, Spectrum has more memory, but when the machine code program is loaded, it was absolutely not uncommon ...
Could it be this one?
Byte's Mac Programmer's Cookbook Paperback – May 1994
by Rob Terrell (Author)
I once had (and enjoyed) it when I still had a Macintosh. Just like the Mac, my issue has unfortunately long gone the path of all obsolete.
And, as an added service:
Apple Rescue of Denver still seems to sell new copies for a whopping $15.
I will answer different question which is assumed by the original question; I am the author of the textbook in informatics, and think answering this way would be appropriate.
It is not that BASIC was great language to program, or people were just writing great books in the past, but now having difficulties with that. Consider the following:
BASIC was one ...
Couldn't find an errata either but you can download the code as a TZX file from World of Spectrum
You can either load that into an emulator and look at the code there (if you want to continue to type it in yourself...best way to learn!) or you can get WinTZX2TAP from World of Spectrum Utilities to convert it to a TAP then get the basic listing using Tappeto ...