I know both authors, so I wrote them to ask. This is a rogue image that appears in various PDFs posted online, but not on Springer's official PDF or any print copy we've seen.

Pretty sure that's a ferret.

oh yeah! that is insane.

I think it's cute!

What if it's steganography or symbolic or a puzzle.

If it's none of that, to complain about it is a bit too harsh.. for something on page 67.