“You can’t judge a book by its cover”—meaning that outward appearance isn’t everything—is a proverb that dates to at least 1929. “Never judge a book by its movie” is a quotation that Reader’s Digest in 1941 credited to J. W. Eagan. The quotation has been frequently cited, but the identity of J. W. Eagan is a mystery.
 
   
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.
     
Google Books
The Reader’s Digest
Volume 39  
1941
Pg. 96:
NEVER judge a book by its movie. (J. W. Eagan)
 
Old Fulton NY Post Cards
11 September 1941, Perry (NY) Record, “From a Library Desk” by Marian Every (Perry Public Library), pg. 3, col. 3:
A phrase in Readers’ Digest advises: “Never judge a book by its movie.” This is very apt advice in many cases, although we can think of some movies that followed the books which prompted their making very faithfully.
 
13 November 1944, Cleveland (OH) Plain Dealer, “The Kernel” by Ed Kuekes, pg. 21, col. 1:
H. T. M. says that you should never judge a book by its movie.
 
24 January 1946, Kingsport (TN) News, “Daffynitions” by Paul H. Gilbert, pg. 4, col. 3:
Book: Part of the familiar saying. “Never judge a book by Its movie.”
 
25 January 1953, Seattle (WA) Sunday Times, “Daffynitions” by Paul H. Gilbert, comics sec., pg. 13, col. 1:
BOOK: Part of the familiar saying, “Never judge a book by its movie.”
     
Google Books
Oxford Dictionary of Quotations by Subject
By Susan Ratcliffe
New York, NY: Oxford University Press
2010
Pg. 82:
Never judge a book by its movie.
J. W. Eagan: attributed; Michael Lent Breakfast with Sharks (2004)
 
Google Books
Tweet This Book:
The 1,400 Greatest Quotes of All Time in 140 Characters Or Less
By Sayre Van Young and Marin Van Young
Berkeley, CA: Ulysses Press
2011
Pg. ?:
Never judge a book by its movie.
J. W. Eagan
   
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Who the hell is J. W. Eagan?
PIOTR KOWALCZYK PUBLISHED ON JAN 10, 2014
“Never judge the book by its movie” is one of the most popular book quotes on the web – but do you know its author?
 
She or he must be a writer. Or maybe a literary critic. A screenwriter? Hollywood-based reporter? A charismatic lecturer or passionate librarian?
(...)
Based on my Google research, the probable answer to J.W. Eagan mystery, is that this is (or was) someone with initials J.W., living in Eagan city, Minnesota.
 
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1:02 PM - 24 Nov 2015