ding-dong
Explanation
This comic plays on the idea of organ donation and the wish for one's body to be useful after death. A person says they hope that when they die, every part of their body is donated, to which another character reveals they've already written a book filling every page of the organ donation paperwork. The twist is that the "donor's" illustrations were too large for the body of the book, so they had to be plasticized and used to educate college students -- turning the person's body into a literal educational resource rather than just an organ donor.
The humor comes from the absurd escalation: the donor wanted to be useful after death, and the other character has taken this to an extreme by already preparing an elaborate publication out of the donor's body, essentially treating a living person as raw material for an anatomy textbook. The joke also plays on the double meaning of "body" (the physical body and the body of a book) and the dark comedy of someone being so enthusiastically efficient about another person's eventual death.