GIMME YOUR WALLET
Explanation
The Joke
A mugger threatens a librarian, but instead of a weapon, the threat is: "Gimme yer wallet or I'll put this astrology book in the reference section!" The librarian immediately caves: "Take whatever you want!" The astrology book is titled "Your Inner Space-Crab." The caption reads: "It's really easy to mug librarians."
The Humor
The joke plays on the stereotype that librarians are deeply devoted to proper book categorization. The reference section of a library contains factual, authoritative works (encyclopedias, atlases, dictionaries), so placing an astrology book there would be a grievous violation of library classification standards -- it would imply that astrology is a credible reference discipline. For a librarian who takes their organizational duties seriously, this threat is more terrifying than physical violence. The book title "Your Inner Space-Crab" is a parody of the kind of vague, New Age-sounding astrology book (likely referencing the zodiac sign Cancer, the crab) that would be especially offensive to place among serious reference works. The humor comes from the inversion of a mugging scenario where the threat is not physical harm but professional outrage.