“Don’t judge a book by its cover” is a popular proverb. As one jocular story has it, a judge dressed up in a convict’s costume for a Halloween party. A policeman thought he was an actual convict and sent him to jail. The moral—“Don’t book a judge by his cover.”
 
“You can’t book a judge by his cover” was cited in newspapers in October 1959.
 
“Never cover a judge by his book” 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.”
           
20 October 1959, Ames (IA) Daily Tribune, “From My Point of View” by Rod Riggs, pg. 4, col. 2:
Foolishness: The police officer, a longtime veteran, had been assigned to a lonely beat in the suburbs. Asked why by another officer, he told his story—A general alarm had been sent out for a state prison escapee. In course of examining passing cars, the officer came across a man attired in prison stripes, and, despite the man’s protests, hauled him in to the station, booked him and tossed him into a cell. It wasn’t until much, much later that the man was able to prove he was a court justice, on his way to a costume party, as a whim, in prisoner’s garb. “Which goes to show,” said the demoted gendarme, “you can’t book a judge by his cover.”
 
31 October 1959, Cincinnati (OH) Enquirer, “Innocent Bystander” by Ollie M. James, pg. A-4, cols. 6-7:
REMINDS us slightly of a jollie in the Wall Street Journal about two policeman talking. They were on a beat way out in the sticks, and one had just been assigned there. It seems that he had seen a judge walking along in a Halloween costume of a convict suit and had collared him and taken him off to jail in the belief he was an escapee. “It just goes to show,” he said, that you should never book a judge by his cover.”
 
15 March 1960, Detroit (MI) Free Press, “The Town Crier” by Mark Beltaire, pg. 32, col. 3:
“I’ve been on this beat ever since I arrested the judge on his way to a masquerade ball,” the former lieutenant said grimly. “How was I to know that his convic suit was only a costume?”
 
“Well,” said the patrolman thoughtfully, “there ought to be a real lesson from that. Never book a judge by his cover.”
 
10 February 1962, Chicago (IL) Defender, “Try and Stop Me” by Bennett Cerf, pg. 18, col. 7:
Patrolman Michael Conlin was banished to a beat in the darkest and dreariest part of town. His sin? he inadvertently arrested a man climbing into a taxi in a convict suit, only to discover that said man was an irascible judge on his way to a fancy dress party. The cop learned his lesson. “That’s the last time,” he swears, “I’ll ever book a judge by his cover.”
 
17 August 1969, The State Journal (Lansing, MI), “The Onlooker” by Bill Sinnott, pg. A-3, col. 4:
Th most unusual loser we’ve heard of was the patrolman who arrested and booked a man he spotted in a prison uniform. The “prisoner” was eventually identified as a judge dressed for a masquerade party. The patrolman was a loser but his experience provides a guideline for all policemen: Never book a judge by his cover.
 
OCLC WorldCat record 
Don’t book a judge by his cover : a collection of the world’s most outrageous puns
Author: Theodore A Brett
Publisher: Santa Barbara : Fithian Press, 1990.
Edition/Format:   Print book : English
 
Google Books
He Who Laughs Lasts
By Shawn Lovley
San Jose, CA: Writers Club Press
2001
Pg. 143:
Then there was the police officer who accidentally arrested a supreme court official who dressed up as a notorious criminal for a costume party. Tsk, tsk: you should never book a judge by his cover.
 
Twitter   
paulkerensa
‏@paulkerensa
Saw a Judge Jules gig ruined by a poor version of ‘Valerie’. Never book a judge by his cover.
6:47 AM - 6 Nov 2009
 
Twitter
Swami Gulagulaananda
‏@Gulagulaananda
Never book a judge by his cover #FlippedProverbs
1:20 PM - 15 Dec 2011
 
Twitter
Tinman
‏@Tinmansplace
A judge dressed in fancy dress as a convict was arrested by mistake..The Police officer was later told.. never Book a judge by his cover…
8:12 AM - 2 Feb 2012
 
Twitter
Lall Anna
‏@lfchero
Two cops accidentally arrested a judge who had dressed as a convict. They later learned to never book a judge by the cover.
6:53 AM - 14 Jul 2012
 
Twitter
Martin Guilfoyle
‏@MartyGuilfoyle
Did u hear about the guard who arrested the judge who dressed up as an inmate for holloween?? Thats why u never book a judge by its cover
9:22 AM - 2 Dec 2012
 
Google Books
In a Manner of Speaking:
Phrases, Expressions, and Proverbs and How We Use and Misuse Them
By Colin McNairn
New York, NY: Skyhorse Publishing, Inc.
2015
Pg. ?:
“You should never book a judge by his cover.” A “lesson learned the hard way” by a police officer. He was disciplined for arresting a member of the judiciary who was on his way to a costume ball dressed as a convict. The officer had, obviously, forgotten the old adage, “appearances are deceiving.”
 
Reddit
A police officer accidentally arrested a judge who was dressed like a convict for a costume party.
submitted September 30, 2016 by Consinneration
He quickly learned to never book a judge by their cover
 
Reddit
A police officer mistakenly arrested a judge who was dressed as a convict for a costume party.
submitted October 9, 2016 by LWKY-XVI
He quickly learned that you should never book a judge by their cover.
 
The Amazingly Lame Joke of the Day
Wednesday, October 12, 2016
A police officer accidentally arrested a judge who was dressed as a convict for a costume party.
He soon learned you should never book a judge by their cover.
Posted by Tim Wycislak at 3:09 AM