Take at least 20 books to a table and work in groups of no more than five people. 

Group the books according to the image on their covers.  Forget the subject and author altogether.  What’s the image?  What colour is it?  Try grouping colours together first.  See how it looks.  It's best to do this quickly, just to get stuck in.  When you’ve tried it one way, try another.  Group the images together - figures on one side, landscapes on another?  Which way looks best?  Try picking books with similar lettering in their titles.

Ask the groups to do this very quickly, just for 5 minutes.

It is difficult to begin with - but it gets easier the more you practice.  Dull covers can be made to look very different when sandwiched between two other kinds of book.