The Vatican is most likely reluctant to open the archives out of a desire to protect the image of the Catholic Church, which is understandable but misguided. The appearance of secrecy does nothing to help their image. At least if they give historians access to the WWII documents we can learn the truth and potentially undermine some of the most outrageous conspiracy theories.
I haven’t read the book yet, but it sounds like a great read and all of the Amazon reviews except one gave it 5 stars. It’s on my reading list along with Rosenberg’s actual diary.
Read the brief article on the book here: Tracking an Elusive Diary From Hitler’s Inner Circle – The New York Times