Prisoner of Azkaban, out August 27, can be pre-ordered here.
In other cover news, Bloomsbury are to release new adult covers for the Potter books, starting with the first three books in the series, which can be seen above. According to Magical Menagerie:
The new illustrations are woodcut images by renowned artist Andrew Davidson. Andrew studied Graphic Design at the Royal College of Art and has worked with a wide range of clients.
The first three jackets will be available from 18th July 2013, followed by the next two on 15th August 2013 and the final two on 26th September 2013. A boxed set containing all seven jackets will be available in October 2013.