I was in a talk that Rinzler did on Saturday in London Film and Comic Con. He mentioned the digital files will be about half and hour long for each book. Video and audio files. He said audio files sound boring but for example there is Alec Guinness practicing how to say "May the force be with you".
It was mentioned about the Indiana Jones book being so small and should have had a book for each film. The powers that be did not think it would sell well as four books so he had to put the lot into one book. He hopes at one time in the future to release an Art Of Indiana Jones book.