Sorry it took me so long to reply Rafael! You can call me dre (my nickname) by the way ;)
The book does indeed bring something new for it focuses more on her career than on any other aspect of her life. If you haven't seen one of her movies, you will feel like you have after reading this book. Every stage performance and TV/movie role is accounted for. The author did his research and gives in depth descriptions of her theatre and movie productions, even going as far as giving his own movie reviews.
The book also gives a good look into her family life (the Kelly's not the Grimaldi's).The book doesn't start off by going into a detailed history of her parents and where their wealth comes from (most biographies I have read about her usually describe that part a little too thoroughly). The author quotes family members and honestly explains her relationship with her family (the Kelly's) better than any other biography I've read. The book also doesn't focus on her love life. Each affair (or alleged affair) is talked about but really the book is not at all gossipy.
Grace first met Mr. Spoto for a book he was writing about Alfred Hitchcock. After that she gave him permission to write a biography about her, if he promised to wait to publish it after she was long gone (25 years gone to be exact). He kept his word and so most of the quotes from Grace come from his personal interviews with her.
I think it is worth reading if you want to know more about her acting career and how Hollywood worked back in the 50's.