Author here, would be interested to hear people's take on the addition of illustrations (illustration is being generous, I know). Do you find them distracting or do they help break up the text?
(Disclaimer: I’m used to Lisp so I’m not really the audience whose opinion matters. But I’m sure I would’ve benefited from reading this earlier. Skimmed it, and am now halfway through a more careful read.)
Images can be great aids in understanding text and making abstract concepts concrete. Also, in many cases the usual metaphor/simile rules apply. For instance, in the phrase he's "fast as a bolt of lightning" we can imagine the speed of the runner and it helps us envision how he "flashes" by opponents. In this case both concepts are known i.e. lighting and fast, but new descriptive meaning is created through the use of simile. Similarly, metaphor takes two known concepts and combines them to reveal a new concept. An example of metaphor, would be "he's an emotional rollercoaster", we know what emotional is and how a rollercoaster behaves, this gives us new insight into the persons high and low spiraling personality.
Essentially, I think good images interact just like simile and metaphor with a text, enhancing, extending, and clarifying concepts. If you go back now and look at your images, I think you'll see that they actually reflect rather verbatim what the text contains, only minimally extend or clarify the text, and at times might make it more confusing (how do turtles enhance our understanding of expressions?). So for these reasons, I found them to be distracting and not helpful, as nice as they actually appear graphically :)
Nice post btw :)