“A book is simply the container of an idea—like a bottle; what is inside the book is what matters.”

Source: Connie Robertson (1998). Book of Humorous Quotations. p. 2

Variant: Boys think girls are like books, If the cover doesn't catch their eye they won't bother to read what's inside.

Source: American on Purpose: The Improbable Adventures of an Unlikely Patriot

Source: The Book Thief

Language Education in a Knowledge Context (1980)
Context: Of writing that is filled with mechanical and grammatical error, as compared with writing that conforms to the rules of standard edited English. Surely, we do not want to say that there is a necessary correlation between mechanical and editorial accuracy and intellectual substance. There are many books that are mechanically faultless but which contain untrue, unclear, or even nonsensical ideas. Carefully edited writing tells us, not that the writer speaks truly, but that he or she grasps... the manner in which knowledge is usually expressed. The most devastating argument against a paper that is marred by grammatical and rhetorical error is that the writer does not understand the subject.

Variant: Books are mirrors: you only see in them what you already have inside you.
Source: The Shadow of the Wind

From the seventh book, "The Book of Youth"
The Pillow Book

Notice a pattern here?
Column, September 14, 2006, "Dems Vs. Wal-mart" http://www.jewishworldreview.com/cols/will091406.php3 at jewishworldreview.com.
2000s