It used to be we could generalize about the differences between US and UK book design. We would claim that US covers were more brash, more “selling”, while UK designers went in for more understated, “beautiful”, “balanced” designs. It doesn’t look like these prejudices have any validity any more — if indeed they ever did. 

The Millions does an annual comparison by C. Max Magee of the US jacket and the UK jacket of the same books: Here’s 2015‘s edition. This is their report for 2014, and here’s 2013‘s. You can go further back (to 2009) by clicking on the link in the first line of the story. They are all interesting, and repay study.