“Don’t judge a book by its cover” is an old proverb. “You can’t cover a judge by his book” is a jocular variation that was used in a song by the Pheremones in 1992.
 
A joke was posted on Twitter by Dave on March 18, 2010:
 
“I saw a magistrate reading a novel, so I grabbed it and put it over his face but he got very angry. You shouldn’t cover a judge by his book.”
 
“Never book a judge by his cover” is a similar joke.
 
 
Wikipedia: Don’t judge a book by its cover
The English idiom “don’t judge a book by its cover” is a metaphorical phrase which means “you shouldn’t prejudge the worth or value of something, by its outward appearance alone”.
 
Early reference
. In George Eliot’s The Mill on the Floss (1860), Mr Tulliver uses the phrase in discussing Daniel Defoe’s The History of the Devil, saying how it was beautifully bound.
. The preceding version was then publicised by the 1946 murder mystery novel by Edwin Rolfe and Lester Fuller, Murder in the Glass Room, in the form of “You can never tell a book by its cover.”
         
17 May 1992, NPR (Washington, DC), “Pheremones’ Sing Their Political Satire,” Weekend All Things Considered:
HANSEN: Here’s “The Proverbial Blues.”
 
J. AND A. PHEROMONE: (Singing) If it ain’t fixed, don’t broke it. With a song on my feet and wings on my lips. Time wounds all heals. Step on a back and break your mother’s crack. What you don’t hurt won’t know you. You can’t cover a judge by his book.
   
Google Books
Groan And Bear It
By Gary Younglove
Fair Oaks Ranch, TX: Piper Plus Publications
2010
Pg. 117:
(A lengthy story. This is at the end.—ed.)
The authorities led the prude away as he protested and challenged the police to tell him what he had done wrong. A sympathetic policeman finally told him that in this community you never cover a judge by his book.
 
Twitter
Dave
@speedyb
I saw a magistrate reading a novel, so I grabbed it and put it over his face but he got very angry.
You shouldn’t cover a judge by his book
3:16 PM - 18 Mar 2010
     
Twitter
Jon Strange 
@Jon_Strange
I saw a magistrate reading a novel,so I grabbed it and put it over his face but he got very angry.
You shouldn’t cover a judge by his book.
9:26 AM - 25 Mar 2010
 
Google Groups: med-jokes
The Judge
chocdr
9/15/10
The Judge
There was once a wise judge who oversaw the judicial affairs of an entire kingdom. He was known far and wide as a fair minister of justice. When he grew old, he decided to retire to the country and let his son take his place at the bench. Before he was to leave, he imparted this news to his son:
(...)
Gesturing towards the tome, the father instructed the housekeeper, saying, “If this ever happens again, remember to never cover a judge by his book.”
   
Twitter
James
@Boskettjokes
I saw a magistrate reading a novel, so I grabbed it and put it over his face but he got very angry. You shouldn’t cover a judge by his book.
8:38 AM - 1 Oct 2013
 
Twitter
🤣 The Dad Joke Man 😉
@DadJokeMan
I saw a magistrate reading a novel, so I grabbed it and put it over his face, but he got very angry…                               
You should never cover a judge by his book…                        #TuesdayTitters #1PUN
1:58 AM - 17 Jul 2018