Guess the theme competition

We're running a new weekly competition in the library.  It's called 'Guess the theme.' Each week there will be a selection of books on the display table in the library and you have to guess what they all have in common.  It could be a colour, something to do with the title or the subjects of the books.  Here are this week's books.  If you think you know the answer you can comment on this blog post or enter in the library.