This almost cliché metaphor basically warns against prejudices against people based on appearance.  If we see a poorly clothed individual asleep in an alley, we shouldn’t jump to the conclusion that he is a drunk, or even that he is there from some sort of neglect.  If we meet an attractive blonde, we shouldn’t assume she is not intelligent.  These kinds of “book by its cover” assumptions creep into our social judgment calls every day.  The essay that examines these assumptions would argue against them, of course, but could also argue for a better way to arrive at such “judgments.”  Expanding on the "book" metaphor here is a thesis statement that could be argued: 

• When meeting a “book” in society, examine its “table of contents,” its “index,” and its “opening paragraph” before putting it in your private Dewey Decimal category. 

You could then go on to describe what it is you look for before judging a “book” by its cover, and what kinds of examinations are legitimate evaluations of human beings.  A good title could be: “How Do I Arrange My Books?”  Good luck on expanding this rough idea.