Collins is relaunching Dr. Seuss's bestselling books, including such perennial favourites as The Cat in the Hat, Green Eggs and Ham and Fox in Socks. In response to consumer demand, the bright new cover designs incorporate much needed guidance on reading levels, with the standard paperbacks divided into three reading strands - Blue Back Books for parents to share with young children, Green Back Books for budding readers to tackle on their own, and Yellow Back Books for older, more fluent readers to enjoy. The 500 Hats of Bartholomew Cubbins belongs to the Yellow Back Book range.