I read a few biographies, official and unofficial ones.
The two i enjoyed most were "A Saucerful of Secrets - The Pink Floyd Odysee" and "How Black was our Sabbath". The last one is written by the personal assistants of Ozzy and Ward and they talk about the 70's. Interesting stories.
The Pink Floyd book is good too, even though the author seems to dislike Roger Waters quite a bit.
My main problem with Biographies from Bands/Musicians is that they rather want to talk about their day to day life and not about how they came up with the Songs, what went on in the Studio. A example is the Book from Tony Iommi, he rather talks about how much drugs and alcohol he did, how many pranks he played on the others who answered with their own pranks (and it was a badly written and constructed book)
Same goes for Run to the Hills, the Iron Maiden book from Mick Wall. Lots of comments from the Band but a bit too few information about their albums, the songs.
And one thing i think is interesting are the mistakes. You should think those people know their lives, but they get facts wrong, dates wrong...Slash in his Biography even gave a wrong date for his first wedding.