As a huge Joni fan, but one who actually not read that much about her, I found this book a great read: very informative, lots of things I didn't know, and some good commentary.
Where it left me very unsatisfied was that it really never got near to appreciating or describing her musicianship and her music. I can only assume that the author isn't a musician, because there is really no feel for this in the book. It's good on the outline of her life, it's pretty good on her lyrics, strong on here art, but weak on the music. And while the wonder of Joni is in part the astonishing combination of her talents, as a musician myself, I feel this book really doesn't do her music justice.
For instance (as a very small example), Joni's ear is obviously so good - does she have perfect pitch? I've always assumed she probably does, but after reading this book, I still have no idea.
Having also just read Brian Glasser's fantastic biography of Joe Zawinul, you see what a good musical biography can do - really get under the skin of the music & the musicianship. This book hardly touches the surface of her music/musicianship.
On the personal front, after all her relationships, Joni suddenly decides to get married. Why? Why him? Why then? I don't know.
I would definitely recommend this book to anyone interested in Joni, and as I say, I enjoyed it greatly and got a lot from it, but the 'definitive biography' is a work still to be done.