Illustrations in my recent books use a font that I purchased years ago for my teaching, Museo and Museo Sans. Back then I wanted something that would look distinctive and eye-catching (yes, I wanted students to read the handout) while being readable and easy on the eyes. The rounded elements really stand out.
When it came to these books I used conventional fonts for the body text of the printed books, Palantino and Courier for code. But used Museo Sans for the figures and covers of both the ebooks and printed books. I did slip in Museo (serif) for the title of The PDP-8 Class Project. Here is an example figure that uses that font:
The much older Designing with Microcontrollers used Times New Roman and Courier for the body text. Figures generally used Helvetica. A few figures from that book were also reused in Far Inside The Arduino, which shows that some concepts never change.