I guess whatever they publish in their books can be used in the re-trial. I wonder how the story has changed between the book and the original story about being stoned, or was it cutting Meredith during an imaginery dinner at his house, or was it failed memory?
It seems similar to a hung jury and an appeal (eg: Jason Young). The only difference seems to be that the Italian courts spend a lot more time in making the decisions ... and the prosecution has the right of appeal.
If the biggest concern about what he wrote in the book is comparing it to something he wrote in his diary and was never used at trial in the first place then I don't think the book will have any bearing during the next trial. And he's never changed his story about being stoned that night. From what I can tell the SC is more concerned about Hellmann ignoring witnesses such as Quintavalle who changed his tune a year after being questioned by police.