The technology for the art is called Wacom Bamboo and I was overdrawing the Erlang logo with some hand scribbles in Inkscape. In fact same technology seems to be used for the original LYSE book and I was using it to translate text in images in Russian translation for the LYSE book.
For Learn You Some Erlang, I used hand-made drawings that I would scan, and then trace over using either photoshop (initially) and then Illustrator (when I knew the book would go to print).
